It sounds like you have been able to resolve most of your problems.
Can anyone give me a hint how to get these back, so I do not have to re-configure everything new, I already forgot all my settings I had until now.
EDIT: Been active, got most settings back.
Unfortunately, I don't know of an easy way to reinstate interface settings from a previous version of the plugin.
n LR6, it shows that Plugin 1.4.3.0 is still active, this I do not understand.
EDIT: A restart of my PC solved this, now I see the latest version.
Restarting Lightroom with the new plugin in place should have been enough (there should have been be no need to reboot your computer) but I'm glad you've got the new version up and running.
-but now, when I leave the WEB tab and go f.e. to DEVELOP, the next time I enter WEB I notice that again the settings are forgotten and I see a "photos" directory to be where my preferred directory used to be
EDIT: Also this seem to have been a setting in my 'old' LR6 and Plugin combination. At the moment this is working correct.
I'm glad that things seem to have settled down and that you have reinstated your settings.
If you continue to experience difficulties, please let me know and I'll try to help out if I can.