Hello,
Ok you have isolated the issue sufficiently. This is a good start. Thank you.
I have tested the plugin with MySQL and it should be compatible with MariaDB.
To further track down the issue, please do the following.
On the server where you can reproduce the issue, please go to the admin dashboard. Under “Bitcoin and Altcoin Wallets” you will find a “Debug” tab. Please let me know what the “MySQL version” is. For example, on my dev machine, I have 5.7.31-0ubuntu0.18.04.1-log.
The plugin takes an integer timestamp and inserts it into the created_time column of type datetime. As you say, it first converts it into a string, here. There should be something in your MySQL server logs regarding this. Perhaps a warning on the INSERT statement?
While you wait for the reply from digital ocean, I will look on the web to see in what server versions or configurations this can happen.
In summary: Please let me know what MySQL version you see in the debug tab, so I can narrow down my search. If you can also check the MySQL log, that will also be helpful.
with regards