When SQLServer is the database updates to the phpbb_config table for "upload_dir_size" can fail, it appears if the value being added to config_value exceeds some value, likely 2^16. The only solution seems to be to use a CAST as a bigint. See the attachment for an example. It appears that the problem is occurring because an explicit SQL CAST function needs to be implemented for SQLServer in the DBAL. Please see the topic where the problem is described in more detail and some patch code I added to db.php for my client to temporarily get around the problem.

https://www.phpbb.com/community/viewtopic.php?f=466&t=2338041