In the last several months Ctrl-B to backup Quicken, reports errors because a "third party" = Dropbox??? has the active file locked. The backup actually happens, but the process generates errors. Then, the backed-up, active file sometime will not load. Note the active file is kept on the computer drive while the backup is on Dropbox.
Hours on the phone with Quicken support has not helped. Windows 11 may be part of the issue.
So far the only solution is: after working in Quicken, let time pass; then attempt the back-up.