I agree, it really shouldn’t.
The WebDAV “(405) Method Not Allowed” error might be the same one mentioned here:
That being said, it sounds like you’ve tried a lot of different methods (thanks!) but just keep running into issues. I know other Synology users are using Duplicati without such issues (at least not that they’re reporting) so I’m stuck assuming your environment or implementation is set up in a way that Duplicati doesn’t handle well.
It would be great if you had the time to help us iron out exactly what the issue is, but without us being able to guarantee results within a particular time frame I certainly understand about the time pressure of dealing with CrashPlan’s announcement.
If you end up finding another tool that works for you please let us know what it is (and what didn’t work for you) in case other Synology users trying out Duplicati run into issues like yours. We’d much rather have people backing up with another tool than not backing up at all!
And keep us in mind once your new tool is in place - Duplicati is actively being developed so at some point we’ll hopefully figure out what’s been causing you so many issues and get it resolved. 