Recently, I performed the Win 10 upgrade operation on my Win 7 machine. Subsequently I encountered printer compatibility problems that resulted in me rolling back to Win 7. Since then, I have been unable to backup successfully; all the scheduled backups have the message "Start Missed-Will Be Run" While trying to solve this issue, I noticed that the drive letter for the backup location had changed, so I corrected it in Computer Management part of Control Panel. Still no success, so I uninstalled and reinstalled Reflect; that also did not solve the problem. I cannot run manual backups either, nothing happens :-(

When you say "rolling back" did you use a Reflect image backup to do a restore to the system prior to your Win 10 upgrade.  If so all should have been put back to the way things were.  If not, what method did you use?

I used the method provided by Win 10 to revert to the version I had before the upgrade. It used a Restore Point that was created just before the upgrade.

Same here after trying out Win 10 and going back to Win 7. Scheduled Backups don't work any more. "Run now" doesn't do anything. No new log entries either.
Opening Windows' Task Scheduler prompts a lot of errors about tasks that have been "tampered with", not restricted to Macrium ones. I'm now running backups from the "Backup Definition Files" tab, then recreating the tasks.

I use WIN 7 and have WIN 10 ready to install but have not done so yet.  If I do a full backup, then install WIN 10, what would be the downside of using Reflect to restore my WIN 7 system? 

Note that I have multiple drives and most of my DATA and programs are on other drives.  So if WIN 10 only messes with the C drive I should be able to just restore the C drive and not lose data that may have come in while WIN 10 was being used, right?  This assumes I decide to revert within a couple of days NOT after weeks of using WIN 10.

I'm not sure what happened with OP.  As far as @Drac144's question I might be able to help a little.  I haven't had any problems upgrading from Win8.1 to Win10.  HOWEVER, I only upgraded so I could use a piece of software to obtain my Win10 key then installed Win10 from scratch.  Before doing so I removed the license from Reflect on the Win8.1 machine and, of course, installed Reflect fresh on the new Win10 install using the key I removed from the 8.1 install.  I've had ZERO problems.

I wish I would have tried restoring the Win8.1 install so I could possibly help the OP.

I have now got a backup running  from the "Backup definition files" tab, as Tjo suggested in his post above. Not a fix yet, but at least I have a way to backup successfully for  now.

@OP
An idea...  Maybe make sure the scheduled backups are running with admin privileges?  Instructions if you need 'em:  Go to Windows Task Scheduler and find your Reflect scheduled task(s).  (They should be saved in the main "folder" called "Task Scheduler Library."  To open Task Scheduler I just hit the Windows key on my keyboard and start typing task scheduler.)  When you find a Reflect scheduled task you're having problems with, right click and choose properties.  In the General tab, click on the "Change User or Group..." button.  In the window that pops up, click the Advanced button.  In the next window click the Find Now button.  In the resulting list pick an admin user account.  Click ok on all three windows that opened.  When you click ok on the last window (the one with the general tab) another window should pop up asking you for the password to the admin account you chose.  Type the appropriate password and click ok.

Beyond that, maybe try creating new scheduled backups from your backup definition files?  Just right click on the xml file in the "Backup Definition Files" tab and choose either Edit or Schedule.

The reason why this may have happened, however, eludes me.  I can only guess.  Do let us know how things go...

While trying to fix this, I have found that 40 tasks are corrupted. I found this program https://repairtasks.codeplex.com/releases/view/617575#ReviewsAnchor that fixed many of them, but fixing the 8 that it could not fix (including all of the Reflect tasks) is proving to be a very time consuming and frustrating job that I seem to have gotten nowhere with despite hours of work. Its looking more and more like I will be restoring the C: drive from a Reflect image in order to undo all the things Win 10 seems to have stuffed up  :-( But that will be for another evening...

I wonder if you deleted the Reflect tasks directly in Windows Task Scheduler then used the Scheduled Backups tab in Reflect to change something (which you could change back) in each backup task to get Reflect to save the change if it'd create the tasks anew in Windows Task Scheduler?  Maybe just making the change from within Reflect without deleting the tasks in Windows Task Scheduler first would work.  Of course, though, if you made changes to the tasks directly from within Windows Task Scheduler after creating the tasks initially in Reflect this method would not be effective because Reflect wouldn't be aware of those changes.