Transferring virtual server makes Mysql on dest. unable to start

I am using “Transfer Virtual Server” to transfer several websites between two virtualmin machines. The source machine is Ubuntu 16.04, and the destination machine is Ubuntu 18.04. The first couple of transfers were successful, but after one of the servers was transferred, the websites on the destination server stop working, and the reason is the Mysql server fails to start. Details are as follows:

This actually happens twice. The first time it happens, on Mysql Database Server page, I see the server is off. When I tried to turn it on, it shows an error message saying “too many connections”, I then deleted the transferred virtual server on the destination machine and made the transfer again. This time the transfer was successful, but I found out something was wrong with the original website’s coding, so I deleted the virtual server on destination machine and made some minor change to fix the problem with the original, and then I made another transfer. This time after the transfer is done, the Mysql stopped working on the destination server. I reboot the machine and it still won’t start. When I try to start it, it says check “journalctl -xe” to get more informations.

I couldn’t provide the outcome of “journalctl -xe” because I have already restore my destination server using an earlier backup. Has anyone encountered this before? Can anyone shed some light regarding why this is happening?

P.S. I will update this post if I encounter this again.

It’s difficult to pinpoint the exact cause if you don’t have log entries and MySQL config provided.