Matomo strange behavior - IP lock by provider

I have a really strange problem. A matomo installation worked well for several month. Than we recognized, that for the last three days there were no visits reported. I thought: “perhaps the update to a newer version will help” - tried it. First attempt didn’t work, second attempt worked (just the update, not the newly report of visitors).
After that, there were some file-leftovers which were recommended to delete. Did it.
Than Matomo told me, that the Database has to be changed to to utf8mb4 chaset. I activated the background task, looked after a few hours but nothing happened. Tracking was green.
Since then (I don’t know the exact “time”) I have the following problem. If I lock in, I see everything. But if I go to the admin section and click something in the adminemenu, my IP get locked immediately by my provider, because of several failed Login attempts, they told me (but I didn’t login).
I have changed passwords, and I made a new installation but with the old database. Same problem.
Is somebody here, who has an idea what this can be? I’m afraid of a hack, but on the otherhand everything started with this update and database problem. Before that, I could navigate in Matomo while searching for the initial problem, that the visitors weren’t reported. Can there an activity in the background which causes this issues? The website I am tracking is made with WordPress.
Sorry, for this long thread, but I try to give all information I have. Has anybody an idea what can cause this problems and how I can solve them? Right now I can’t do anything in matomo because the IP is locked before I can do any admin work.
Thanks in advance