Windowed mode even with FULLSCREEN=1 in skyrimprefs

[Valera512]

Version of Skyrim Together

v1.3.2

Description

I guided a friend through the installation of Skyrim Together.
Fresh Skyrim Anniversary Edition, No CCC, ModOrganizer2, newest SKSE plugins, everything identical. For me, everything is working fine.
His SkyrimTogether however starts windowed, with a green outline and most of his keys don't even seem to be registered by Skyrim Together. Normal SkyrimSpecialEdition works fine. Our first guess was to start SkyrimTogether in fullscreen mode.
So we tried changing the settings by

• Starting SkyrimLauncher and SKSEloader.exe afterwards (both are starting in fullscreen mode)
• editing the skyrimprefs.ini with MO2
• editing the skyrimprefs.ini in MyGames/SkyrimSpecialEdition, as well as in steamapps/common/SkyrimSpecialEdition manually
  Nothing seems to affect SkyrimTogether.

##System
Acer Aspire E15
Intel Core i5-4200U
Nvidia Geforce 920M, 2GB VRAM
Laptop resolution: 1366x768

Reproduction rate

Please check just one of the following reproduction rates:

• Consistent: I can reproduce this bug by following the steps to reproduce 100% of the time (every time)