I might have the same problem. Any time i start a backup, regardless with what settings, it never starts. I do not open another thread as i guess it is the same problem.
I just shows the loading animation on the ok button and that's it. Cancel is greyed out and doesn't work.
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When this happened the first time and i could not stop the process i had to restart the server. The second time i just left it over nicht to see where it ends up and that is the result. Seems to me that plesk cannot load the small info box that it is performing a backup or something like this. This msg appears after long time (i cannot say how long exactly as i was going to bed...)
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The reallly bad thing about is, that even the scheduled backups did not run in the night so my data is unprotected for loss.
I will try out the reg option to get a dump and reproduce the roblem after. this might take a while since it takes hours until the error msg appears.
My config:
Plesk 17.0.17 Update5 on Windows 2012R2 x64 on actual patchlevel.
Thx in advance