IMPORTANT NOTE:

Square made some changes on March 13, 2019 which change how the module authorization tokens work.
You NEED to update your module to version 0.97 (just released today) in order for your token refreshes to work properly. And new installs won't work if you're using an older version of this module (ie: 0.96 or older).

This 0.97 update includes TWO important factors:
a) the square_handler.php update mentioned a few posts back (#264 on Feb 28)
b) a forced (slightly older) API Version header sent with each transaction (so you don't have to override it manually in your Square account)

Fortunately, if you were already using 0.96, updating to 0.97 involves only updating 2 files. See details in the module download at: Square Payments for Zen Cart