You could try using forum search or (maybe better) an external search engine to see if you can find any similar purge-broken-files stalled-running cases. I tried earlier (but only to see if it suffers from the high-resource-usage-to completion that Recreate suffers from). It seemed like it ought to be finishing faster.
Another possibly slow option is to use the Database option to find its path, rename it, and try Recreate. Coming back to debug --purge-broken-files further would then still be an option if Recreate won’t finish.
I don’t know how likely it could be that most of Duplicati would work, with this not working, but reinstalls sometimes help. Here, Windows install without prior uninstall can break. Duplicati files can be missing.
Mono has some tracing options, but it’s been a long time since I tried them. In addition to strace (which doesn’t show anything at all going on?!), you might have ltrace if you really want another view of things.
Duplicati code is, as far as I know, largely the same code on different OS’s, with some OS differences such as which way the slashes go, low-level things like file accessing, and a few OS-specific features.