I was enjoying our test server when I was asked by an admin "what are you doing?, the server is throwing up tons of errors"

I was simply placing stone bricks in the nether at the time, but it was in overworld as well as nether, and with breaking as well as placing. I was asked to re-log. So I did.

I haven't been able to log back in since, getting "Internal Server Error" every single time, for over 24 hours.

I have tried uninstalling and reinstalling both the game and java. Restarting computer, turning off PC completely and back on, malware/virus scan, CCleaner scans, even a Disk scan to see if my HDD was throwing errors - and nothing. I then tried restarting router, even turning it off for 5 minutes and then plugging it back in. I checked my NAT settings, firewall (on router) and nothing had changed or been added. I removed some custom profiles I thought were causing issues, but nothing changed.

I've been using discord talking with the server admin and even had another person log into Minecraft on my PC, with their account -and they were able to access the game instantly.

What on earth could be causing this problem?

To make matters worse, this is the third "high end" problem I've had with snapshot release. Twice already, in nearly the exact same place, doing nearly the exact same mechanics (running, stopping, eating, then swimming) I have crashed, only to have my data corrupted throwing Java errors (not "Internal Server" like now ) but we've managed to work around those. Something is very wrong here, as there are NO addons, modifications or anything on either Server side or Client side, not even texture pack or custom/renamed sounds.