Thanks for the response, Hubert.
Of course I didn't just ask this question here. I also asked some unix / linux-geeks among my friends. One of them answered with this
link (shame on me I didn't discover
I don't know what the problem is / was, but now the plugin runs. No charts
so far, but a re-import might solve that. Some other things not working OK - the upgrade 'broke' some other software
Pavel made a remark about different GCC versions. Apparently Pavel compiled with the 'new' GCC. If that's really the case, well, maybe make a remark on the o-charts page - would have saved me a lot of time researching
For your other question: going back to 14.04LTS would probably mean a fresh install. Of course, that's possible. But it will cost some time and somehow I have to do the upgrade again - as other software
requires 15.10 or higher.
Maybe someone can profit from this. Now let's get the charts visible. And see how stable the system is...
Thanks so far - will report later about charts (or no charts, depending on the outcome of the experiments