Update: found this error in payment provider's log file, seems to be the cause:
2022-04-05 13:46:25.915 - SQLite error: UNIQUE constraint failed: Payments.CheckSequence, Payments.LongToken
2022-04-05 13:46:25.915 - SQLite step result: 19
2022-04-05 13:46:25.915 - SQLite finished executing
2022-04-05 13:46:25.915 - Adding payment details
2022-04-05 13:46:25.915 - ApplyPayment::getTenderMediaObjectNum()
2022-04-05 13:46:25.915 - Fallback to BrandTenderMap Singleton
2022-04-05 13:46:25.915 - Tender Media Object Number: 202
2022-04-05 13:46:25.915 - approval code added: 861724
2022-04-05 13:46:25.915 - ApplyPayment::setCardDataFields( <payment> )
2022-04-05 13:46:25.915 - Entry method: 'S'
2022-04-05 13:46:25.915 - Modifying Check
2022-04-05 13:46:26.876 - Check Modified
2022-04-05 13:46:26.876 - Error processing threaded call: *PLEASE SWIPE ON PINPAD
2022-04-05 13:46:26.876 - SQLite finished executing