Strange behaviour of different clients using same OpenVPN account

[Scholler]

Hi all,

in my OpenVPN config I have some users each using a different subnet (via overrides) to allow traffic to specific destinations only.
Authentication using client certificate, username and password.

Now I have a phenomena with one user.

If he uses his exported client certificate together with his OpenVPN client on Android, he can login successfully AND work on his allowed destination machines.

If he does the same on Windows 10, he can also login successfully BUT there is no traffic at all to the destination machines. I don't see anything in the firewall logs.

All IP addresses / routings correctly are correctly set in both cases.

If the user tries it on the same Windows 10 with another account (cert file, user and password), he can reach to his destinations.

What the hell is this? I do not have a clue. Anyone able to help? Thank you.