GENERIC_DECLINE is the square equivalent of "oops!"

"An unexpected error occurred" is certainly not a lot of help.

First, make sure your API is dated 04-22-2020.

Then, if the module's Log Mode is not set to notify you of errors, select at least log on failures.

Next, try dropping the PHP back a notch just to make sure your 7.3 is not just outside the current range of 1.5.6c

Just in case, check server logs for the times these failures occurred. With everyone using the internet at home, you may have some dropped connections.

Keep in mind, that some of these transactions might be someone just testing to see if a hacked card will work.