Here we go:
I set my Client requirement to 20 and my server requirement to 10.
I was not able to use the Item with level 10 on my server.
When I set the Client requirement to 10 and the server requirement to 20,
then the server thankfully prevents me to use it at level 10, eventhough my client visually says that I should be able to use it.

Another Problem is that when I join on a vanilla server that does not have reskillable
installed then I am not able to use any of the Skill Locked Items because I am
unable to Level UP!

The issue has to do with things not properly being synced, which will be fixed in 1.15 with the rewrite of this mod as Epos.