Today, before lunch, I decided to investigate the issue again. I checked the FritzBox to see which devices were connected via WiFi. After lunch, we started debugging the problem again.

To our surprise, the remote connection worked perfectly!

I reviewed the list of WiFi connections and noticed that three items labeled "wlan0" were no longer connected. Long story short (and I'm so happy we finally solved this!):

The issue was caused by three "prios smart lighting" light bulbs!

• I confirmed it by going into my office and turning the lights on.
• Immediately, my girlfriend's remote connection started lagging again, and after a few seconds, she was disconnected!

What is going on here...?

We both noticed a pattern: her remote connection was often bad during bad weather. Naturally, I had my office lights on during those times.

I hope this discovery helps others experiencing VPN disconnects or poor remote PC performance while working from home. And if anyone can explain how these "smart" light bulbs are causing this issue, please enlighten me!