The ticket I submitted yesterday has been solved, somewhat. The culprit was another plugin. However, I had to deactivate the plugin on two websites in order to make them accessible again. I also deactivated the plugin on all other websites for the time being. There is an issue with the plugin, that’s for sure.
I just noticed that there are three active licenses of the collaboration plugin in my account. How come? I thought I only bought one. At least I only paid for one. Is that maybe the reason for the problems I’m facing with the plugin?