I fixed the problem. It was not my connection that was the problem, but my wallet.data file was corrupted. Here’s what I did for toubleshooting (with the help of a friend):
I did the following while using 3 different laptops:
I started by removing the litecoin.conf file in the app data folder. I rerouted my router port to 9333 by creating a new connection via a TCP port. (Windows firewall).
Laptop#1: This is my laptop that I normally use for the LTC wallet. I removed the wallet including all related files and installed a new wallet without putting back the wallet.dat file with all my coins. So this wallet had it’s own wallet.dat file and showed the same network connections (In:0 / Out:8) as the other 2 laptops. (So, no litecoin.conf file anymore)
Laptop#2: Had the LTC wallet with the wallet.dat file with my coins. (No conf.file)
Laptop#3: Had another LTC wallet downloaded some time ago.(No conf. file)
Laptop#1 and #3 could send and receive coins from each other. (No conf. file)
Laptop#1 could only receive coins, still couldn’t send any coins. I was now suspicious that it could be my wallet.dat file, not the network connection.
There was a difference in the amount of files in the appdata folder when I compared the wallet with the wallet.dat file containing the coins to the other ones.
This app data folder also contained a fee.estimates.dat and mempool.dat file which seem to appear once I opened the LTC wallet with this wallet.dat file.
Then a friend suggested to open the wallet where I had all my coins and perform a rescan of the blockchain via the debug console panel. It took some time, but after this rescan, my wallet worked again and I could send coins again from this wallet. I did lose some coins, so I have to figure out now where those coins were lost. I think it could be that these transactions were corrupted and never reached my wallet in the first place. But I still have a good amount so I’m happy about that. I transffered all the remaining coins to a hardware wallet. I will never use the litecoin core wallet again.
Hope this info can help someone out there with the same issues.