Up until now, I was logged in continuously for years and was at least able to post and comment.

How were you logged in continuously for years?

For example if you are using Hive Keychain the keys are right there in your browser extension and you can easily extract them. Of course Keychain does not store the owner key which is pretty important for "owning" your account, but all you need is the "active" key to powerdown and move that value to a safer place, which keychain might have... assuming you use it.

master key and activation

The master key can't unlock anything on Hive.
It can only be used to generate your other 4 keys.
If you have the master key you need to use it to generate the other 4 keys.

key-generator

If you have a master key go to this website and type in your username (Hive account@erh.germany I presume) and below that the master key. This will generate from scratch 4 other keys that might work if the master key is correct. These keys change based on the username so make sure both are correct.

https://hiveblocks.com/@erh.germany/~owners

This website shows that your owner key has not changed (at least since 2020).
The chance that you have been hacked is very low.

Owner key

If you have a valid owner key it can be used to change both the active and posting keys.
I think technically the owner key can be used to sign all operations but some frontends can have trouble with it so it's better to check if you can change the keys. Of course it would be unlikely that, given 5 keys, the master or owner would be correct but the active/posting/memo would be incorrect. Still worth a try though.

That's about all the advice I can think of at the moment.
If you're close to giving up you might consider direct messaging me your private keys on Discord so I can personally take a look at them.