Every 30 seconds I get a few (2-4) lines like this:

[08:58:38] [Server thread/FATAL] [SolarFlux/]: Refreshing upgrade cache.

This is after upgrading from 0.5b to 0.6 on a world which had existing solar panels.

Forge Server on Build-1403

Thanks a lot for taking the time to report the problem. It is a debugging statement I forgot to remove. Nothing to worry about apart from the fact that it will fill up your log. I will upload a new version in a couple of hours.

Pushed version 0.6a on Curse.
Just for your info, this log statement was not reporting an error. The mod works perfectly fine. It was just the log level that was set to FATAL instead of DEBUG.