Sorry for the late reply, had to verify on more than 1 PC.
At first, yes it was the active Defender! (in conjunction with the security update and Eset)
Typically ESET deactivates the Defender on installation and everything is fine, I don't know why and I can't tell atm for sure which case is correct:
- Defender was still running on these few PCs, never had a Problem until the Security Update changed something
- Defender was not runnung but the Security Update activates it for whatever reason, bug, ...
I'm not done fixing all PCs, maybe there is one PC left to find out which case is correct.
Thanks to all!