I had this issue on two Beelink Mini MXIII's after upgrading from kszaq's 7.0.3.012e Jarvis to 7.90.beta3 Krypton. Both had 10 second wait for network set in LibreELEC settings. Both are connected using Ethernet.
I tried all different ways of getting it to work and every once in a while one device would connect to the MySQL server but then drop out again. The MySQL server is MariaDB running on a Synology NAS. I thought it may have been a permissions issue and/or NFS issue but changes to either of these on the Synology did nothing to fix it. I also tried 7.90.beta3 installed both on SD card and NAND but this didn't fix it either. I changed wait for network to 25 seconds, didn't fix it.
In the end I reverted back to 7.0.3.012g Jarvis and restored my backups and the MySQL shared library works fine again on both devices. My logs are identical to the ones pasted on the original post:
ERROR: Unable to open database: MyVideos107 [2003](Can't connect to MySQL server on '192.168.1.50' (101))
etc. etc.