Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2006 MySQL server has gone away'
99% of the times I've seen that error it is because the website is trying to save more data than the amount expected by MySQL.

It is fixed increasing the value of the mysql variable max_allowed_packet

The value of this variable is given in bytes, and by default it is set to 1048576 (1 Mb).

You can try to modify it to something bigger, like 4194304 (4 Mb) and keep increasing it to a value that do not give you this error.