I encountered this bug working with a client and fixed it with the solution in the last post of this topic:

https://www.phpbb.com/community/viewtopic.php?p=15011511

I suggest that the field allowed_schemes_links and similar one line fields in the ACP where comma-delimited lists are expected should do some clean up before saving the field in the database, or interpreting it when read. If a comma is in the last position, it apparently gets interpreted incorrectly. I'm guessing a PHP explode statement is executed, and content after the last comma is read into an array, and that's tripping up the TextFormatter library, causing the error.