Which version 2.0.. or 2.1.. should be considered?
I'd go with 2.0.x. 2.1 is still in beta.
Is there anyone who experienced similar difficulties?
Yes, I had the same issues when upgrading from 1.1.16 to 2.0.15. Errors, errors and more errors. Also tried for years and each upgrade I got a little bit further, but I finally upgraded the forum this year (in February I think). And, just as in your case, I also inherited a forum, so I know the situation you're in. The original admin is probably of no help... he either forgot how or what he configured/changed or he's unavailable

... well, at least that was the case in my scenario.
Follow
this "tutorial" I wrote in another thread. Other users may have written similar ones (search the forum), but this is what worked for me. Kindred corrected me in some parts of the tutorial, so read the whole thread.
Good luck

.