The issue has been present for about 10 days, before it has been working for over a month.

We have noticed now, that FFXIV is not the only game affected though.
I also know of another person owning this exact mouse, who does not have any problems with this key.
After what we tried so far this is all very confusing.

What we tried so far but nothing helped:
- Update drivers, software and windows
- Remove and then reinstall Mouse driver
- Repair and Remove Razer Synapse (including deleting all files)
- Removing all Profiles and create new ones
- Allow Synapse through Firewalls etc


- Thanks for any help -