Network Frame Drops after a certain time of streaming to Twitch

I have a VERY frustrating issue for more than a month now:

After a certain time (which is about 1,5h) of streaming to Twitch, the connection to Twitch becomes very unstable and OBS starts dropping network frames at a rate up to 90%. restarting OBS or the PC and trying to reconnect does not resolve the issue.

What I've done to resolve the issue:

1. checked for any applications using much bandwidth
2. checked OBS config and tried various config changes
3. running OBS as Administrator
4. updating Nvidia Drivers
5. running OBS without remote debugging
6. restarting OBS while the problems occur
7. restarting the machine while the problems occur

Everything aside from OBS keeps working normally.
Reinstalling the OS is NOT an option for me.

Further information:

1. I'm not using Wifi, only ethernet
2. there are no devices in my network consuming any bandwith worth mentioning
3. the connection to the router was tested several times

A OBS log file from today is attached.

I've also done several speed tests at various times. I know that those tests not always deliver accurate data. Although, the Cloudflare speed test showed a high jitter rate and very poor upload (1Mbps instead of the 50Mbps I normally get) when I once ran the test right after I had to cancel a stream after the problem occurred. I've attached a screenshot to this post.

A technician from my ISP also already came to my house and replaced my plug socket after detecting problem with the upstream which disappeared after the replacement. He also stated that it is unlikely that the problems he detected are the cause for my problems which I explained to him in detail.

I don't know if the behavior recorded by the speed tests is really related to my problem.

The uploaded OBS log is from after the replacement.

Speed tests don't measure RTMP protocol which is used for streaming.
If in a normal speed test you have packet lost, then you need to contact your ISP regarding this. There's an issue they need to solve.

15 plugins. Check for updates and compatibility with OBS 30.2.x
Remove all that haven't being updated, the ones you don't use or are simple not relevant for a good stream.
If you do need some of the plugins and are still not updated, downgrade OBS until this plugins are updated.

I've forgot a very important detail. This is what makes the whole issue pretty weird:

Whenever I start a stream using the bandwidth test mode, the connection never had a single issue.
I tested this for many hours now.

I ONLY encounter problems when streaming to Twitch for around 1,5h.

Also, as I already mentioned, my ISP is informed and the technician already did what he can.