Google tells me that Maria DB "is a binary drop in replacement" for MySQL, and that the MySQL client also works with MariaDB server - so it should work with QSO Relay if that is the case.
In order to get the performance to an acceptable level, the synchronise database operation truncates the 'Log' table in the QSO Relay database, and then inserts all rows from the HRD Logbook as a single transaction. What this means is, if any one of the rows being inserted causes an exception, the whole transaction rollbacks and the 'Log' table in QSO Relay will be empty. So that is why you see no contacts in QSO Relay.
That exception says in part: "Input string was not in a correct format. Couldn't store <> in RxPwrDecimal Column. Expected type is Decimal."
You might want to take a look in the database and see what value you have in the Rx Power (COL_RX_PWR) column for a contact just before, or near, a contact with UT2IJ on 2014-05-03 at 00:11:38.
The insert statement reported in the exception log is for this contact, and is a valid statement. But the row that actually caused the exception, may be the row just before this one in time (the rows are selected in QSO Date and Time On (COL_TIME_ON) order descending during the sync).
It is weird though, as COL_RX_PWR is a double/float type in all platforms supported by HRD Logbook, i.e. Access, SQL Server and MySQL, which can only hold valid floating point numbers or null.
Unless the Rx Power column value is important to you, try setting the value to 0 in the suspect row or rows and see if that allows the sync to run without exception.
Alternatively, you could send me a Maria DB database backup and I can restore it here and see if I can replicate the issue. I will download a copy of the server from mariadb.org to my development system.
Let me know how you get on.