Had my character walking half in the ground in the last boss fight on the first stage, in the lower left area. While i could get back up, the legs out of the ground, when moving back to the lower left area the same happened again in that session. While i had that situation like 3 times, the other times i had to fight the boss (yes tough fight), the situation did not occur again. So like 3 out of 6+ times.

I can force this to occur in Stormfall by approaching a gate and not releasing my mouse button to click the gate. The character will start sliding then fall through the ground after a short bit of hopping

This does occur from time to time yeah :D I had a regular sadistic thug, vanishing from the map as I was about the fight him.