Can customer payments be edited after save, at all? Our accounting team needs to change the date on several customer payments. Editing a customer payment throws "Please enter a payment amount or apply credits/deposits" and won't save. I get the same error if I try running a workflow or CSV import to change the date too. Even tried just opening the record in Edit mode and hitting Save and got the same warning.

Is the accounting period closed?

Period is still open. If I make a "fake" $0.01 line item change it will save, but then is off by $0.01 in the Applied v. Unapplied.

Sounds like you have a script running on the record. That blocking your save. Look at Customization >Scripting>Scripted Records > customer Payment and start the debugging process

There are scripts, but none of them have this error script. Our final workaround was to enter the amount as $0.01 and apply a correcting journal entry to make it all balance out. Other options reviewed were to export, delete, and then import or re-key.

It does sound like you've a script or workflow firing off doing something that's causing the error