This is exactly what happens.
Looks like you are not the only one with this issue. But, I believe I fixed it. Deleting cache.db in your Blockland folder fixes the problem. The validating issue finally vanished when I did that.
If you are running non-steam Blockland, cache.db is located in your Documents/Blockland directory.
Steam - Computer/OC (C:)/Program Files (x86)/Steam/steamapps/common/Blockland/
Before I deleted cache.db, I had to always re-insert my authentication key multiple times and it fixes the problem, but I had to do that every day.
Try the first step, if it doesn't work then resort to the second step.