[HeresJohnny]

Returning to my MySQL problem.. I did a bad thing. Wanted to update from 5.7.19 to 5.7.21 and had forgotten my root password. For kodi, there was a special user with admin priviliges so I tried to reset the root passwort and shot the database. Deleted everything and started with 5.7.21 from scratch. Unfortunately that means I can do no more tests with that database.

I have saved the complete old config, except the value for query_cache_size which I don't remember exactly but which was something between 80M and 100M (because I had read this article https://haydenjames.io/mysql-query-cache...rformance/). In a comment on the user manual for query_cache, Hayden James states: With the popularity of InnoDB as of MySQL 5.6.8 query cache is now disabled by default. Previously I recommended a small query_cache_size here: http://haydenjames.io/mysql-query-cache-...rformance/ However, with InnoDB it's almost always better to disable query cache now. Which is probably part of the reason for the new default.

Here's the config of my.ini
https://pastebin.com/G86KesjM

There are actually two ways to disable the query cache which are
query_cache_size = 0 
  or
query_cache_type = 0

The manual recommends:
If query_cache_size is 0, you should also set query_cache_type variable to 0. In this case, the server does not acquire the query cache mutex at all, which means that the query cache cannot be enabled at runtime and there is reduced overhead in query execution.

When I reported that I could now boot into KODI I had this config
query_cache_size = 0
query_cache_type = 1

Just for completeness sake, I run MySQL on Windows 10, 8GB, Intel Celeron 2930. There's also a mail server running on MySQL.