tags not being saved when editing

Hi, I can’t find a solution in the existing forum posts so here is my problem:

When editing the tags of a all new or also an old post, some of these tags are gone/not saved after saving/actualizing the post or if you want to call it differently they disappear. I tried in in the post edit view and in the quick edit list of all posts. I have no adblockers, javascript blockers in my browser(s) and I disabled ALL plugins in my wordpress installation. I logged off and on. I changed the permalink settings to standard (?id=). The most weird thing is, for some tags it works like it always did and for some not. No dashes, no special characters. I can’t understand it.