I have Macrium V8.0.6635 scheduled to run full backups on Monday night and differentials the other nights until the next Monday.  I'm doing image backups from my PC to my Synology NAS. On Monday the full backup ran fine, on Tuesday the differential ran fine and on Wed. it attempted to run another full backup.  The messages in the log were:
Full - Differential specified but no set to append to
File Name: \\DS918\Backups\ASUS N550JK\Macrium Reflect Differential\8EFC2F5A7EE801BB-00-00.mrimg
Attempting to connect to: '\\DS918\Backups\ASUS N550JK\Macrium Reflect Differential'
WNetCancelConnection2: '2250'
These were followed by the following messages:
Failure - User - xxxxxxx - Multiple connections to a server or shared resource by the same user, using more than one user name, are not allowed. Disconnect all previous connections to the server or shared resource and try again.

I rebooted the PC and used the Scheduled Backups tab to run the backup, which ran successfully. On Thursday night the differential backup ran fine.
I have seen the multiple connections message previously. Did the multiple connections error message cause the scheduling problem or are both messages separate issues?

The Backups shared folder also has other subfolders.  One subfolder is used for Quicken backups and two other subfolders are used by SyncBackPro to backup photos and other data from external hard drives.  None of the other backups were running at the same time, but may have run earlier in the day.  Could the multiple connections error be caused by SyncBackPro and Quicken using the same share folder?

Thanks you any help you can provide.

It's not clear from your description how the backups you mentioned before seeing the "no set to append to" message were launched.  Scheduled backups, including manual invocations of a scheduled task under the Scheduled Backups tab, run under the context of the user account specified under Defaults and Settings > Schedule, which by default is the Windows built-in SYSTEM account.  As such, in order for a backup running in that context to access a network resource, it must have credentials valid for that resource, which are stored under Defaults and Settings > Network.

By comparison, backups started interactively under the Backup Definition Files tab would execute in your own user context.

The multiple connections error means that within a given Windows user context, you cannot connect to the same network resource with two different credentials simultaneously.  So if something else you're using is running under the SYSTEM account and connects to the NAS using different credentials, then that might cause that error when running scheduled Reflect backups.  Similarly, if you have something running within your user context that connects to that NAS and a Reflect running within your user context tries to use other credentials, that will also cause that error.  Sharing folders by itself isn't a problem.  The question is what credentials are being used to connect to the NAS.

The backups are launched as scheduled backups.  The only time I use the Scheduled Backups tab or Run under the Definition Files tab is after I've experienced an error with the scheduled backup; such as a network error.  I do have the credentials for the resource stored under Defaults and Settings>Network.

Prior to using Macrium, I used Acronis True Image and had different credentials for the shared folder.  When I stopped using Acronis this past June, I deleted the subfolders used by Acronis and deleted the credentials for the shared folder.  I've also rebooted the NAS a couple of times.  I haven't created any other credentials for the shared folder, and I'm the only person who uses the NAS.  I setup my wife's PC to backup to a different subfolder under the same shared folder, and I used the same credentials for her backup.  I'm confused as I don't know what would be using different credentials.

I encountered the problem again on Saturday night.  A half hour later I rebooted the PC and ran the backup from the Definition Files>Run tab, and it ran fine.  Yesterday the backup ran without problems.  I have how to view connections to my Synology NAS, so I'll try to check before each backup. It is interesting that my wife's PC doesn't this problem. Her backups go to a different subfolder under the same Backups shared folder.

Today I rebooted my PC and checked Synology to make sure there were no connection to the Backups shared folder.  In Macrium I went to Other Tasks>Edit Defaults and Settings>Network Share Settings and selected the credentials for the Backups share. I clicked on Test and received the multiple connections error.  I've attached a screenshot.  I didn't think that was supposed to occur, and I don't think it occurred when I added the information.  If it isn't supposed to occur, I probably have something clobbered in my PC, and that is the cause of the random errors. I'm not sure how to find out what is wrong.

I opened a ticket with support on this issue, and they suggested that I use the IP address of my NAS instead of the server name.  The last two backups using the server name ran fine and last night's backup using the IP address also ran fine.  Support also had some suggestions for releasing the connections prior to a backup.
Yesterday I noticed that my wife's PC backup didn't disconnect two hours after the backup completed, but it had disconnected when I checked this morning.  This morning I ran a Quicken backup to the Backups shared folder and monitored the connection from Synolgy's Resource Monitor. It showed the connection being made and disconnected about a minute after the backup completed.  I've asked support if Macrium might have a problem disconnecting after a backup completes or if they are supposed to do the disconnect.

Interesting findings.  I don't think Reflect closes NAS connections, and that may be by design because in some use cases, Reflect backups might be invoked by a script that might be configured to do something else at the destination after the backup completes.  In fact Reflect itself offers capabilities along these lines.  It can generate a script in BAT, VBS, or PS1 format, and in the latter two varieties, it includes an option whereby a user can choose to have the contents of the destination folder replicated to some other location after the backup completes.  The replication itself is achieved by using the Robocopy utility built into Windows.  But if the "primary" destination folder is on a NAS, then the network connection that Reflect opened to put a backup there will need to remain open for Robocopy to access it afterward.

@jphughan
Support confirmed that Macrium doesn't perform a disconnect, and some of the things you mentioned are their reasons for not performing a disconnect. They did provide a KB article that provides some options for doing disconnects.  Here is a link to the article.
https://knowledgebase.macrium.com/display/KNOW80/Network+Share+Persistence

The article may be incorrect in stating the Windows time-out value is 15 minutes.  The connection for my 7PM backup still exits over 12 hours later.  I don't have anything that I'm aware of that would have accessed that share folder during the night.  I'm going to check to see if that value has been changed somehow on my PC