I have 2 computers both running Ultimate V2018.01 and have been syncing with a MYSQL (FreeNAS Jailed) server for at least a year now, no problems.
Had to shut down all computers and freenas server due to some heavy thunder storms last week (7,000 lightning strikes in one night in out reign).
After the storms I powered up the computers but left the freenas off as there were some more storms due and the NAS server sits in my workshop outside so a pain to just switch back on.
Now I know the 2 calendars would not sync but I thought not a problem, after I power up the NAS everything will sync -

Powered up the NAS a couple of days ago and both calendars did not sync and the desktop (full) calendars on BOTH pc's showed Server offline.
Click on the bottom right - RED server offline and a window comes up - "unable to connect, A problem occurred while attempting ...etc - Invalid field in source data:0"
Under sharing - press test connection and both say connect OK
Did not have time till today to look deeper but now one PC "appears" to be connecting (I've not done anything) but the other still no go
Tried restarting the MYSQL server and the whole NAS server and no change
I can use HeidiSQL to talk to the MYSQL server OK and see the various tables
(BTW should the transactions be cleared (1,318 Rows - 192KB) Timestamp from 15/8/2017 to 13/01/2018
On both PC's under sharing, the view log and sync now buttons appear to do nothing - I wish there was some more information displayed, especially when hitting the manual sync now button
Can't find any log files in Vueminder program files folders
Help please as I now have 2 calendars that are out of sync
Leigh