An error message occurring for all of us that are using it, including system administrators. It is occurring every time we enter a line into a batch, not just sometimes.
Step 1: We recommend every user of Payments2Us be assign a Permission Set. See Step 5 of: https://help.payments2us.com/m/installation/l/824443-how-to-set-up-internal-security-salesforce-users#assigning-permission-sets-to-users
Step 2: Another thing to give a try at least is to different browsers as well. Particularly if its just one user.
Step 3: Then if still having issues, suggest doing a test with DEMO Facility and Payment Form. These are the ones generated by create samples.
Step 4: Then, if still having issues, try turning off any flows, process builder, validation rules on the Batch Entry Line Item or Payment Txn. Do another test and if still occurring, re-activate it.