I haven't had the same level of difficulty that you have, Bent, but I have written to Diigo about some concerns that I have and the most sufficient thing that has happened is that they have given no response at all.
This is disturbing and, if I were in your shoes, Bent, with the same things happening to me, I'd feel the same as you do.
I haven't put a lot of time into Diigo either, but I'm now beginning to think that I won't.
I'm from Arkansas, went to school in Texas and California, and now I'm a retired music professor at SDSU after teaching there for 40 years. I'm working on a post-career web project of my pre, career and post-career interests at http://www.jackdlogan.com/. Come on over for a visit.