I have no specific idea about your problem, 2 quick notes:
you seem to be trying different options in the last task creation tab, but always getting the same error message, the save on this tab is not specifically linked to the options on this page, it’s a global save so the problem can be linked to any of these 5 pages (or none)
it seems that Duplicati is running a backup while you are creating a new one. I have never tried this, maybe it’s related ? Duplicati server may be in a blocking state or whatever.
If the problem still appears after restarting Duplicati, I’d try to stop the server and restart it explicitly in a command windows, I go to the Duplicati directory and starts it with the same options as the service (if you use a service), along the lines of:
mono Duplicati.Server.exe --webservice-interface=any --webservice-port=8200 --portable-mode
Looks like its simply error during backup configuration. That empty error box is an issue for one.
The following is definitely the function there as removing name shows the error for that. It does validation so if something is configured wrong or something else in a way that wasn’t noticed then it might go wrong like this. But, I’m not sure where its traveling in there. It could use debugging. For normal users, maybe the logs might say something?
You might also be able to figure it out based on the code. Rule out each thing it validates and might find something. Otherwise, something is changing how it works in an unexpected way.