This can happen if you don't have write permission on the custom folder you're using in-game. Sometimes this can happen because an anti-virus or other program is blocking writing to a specific folder.
There are a few things that can help. One is to run OOTP in admin mode. Another is to whitelist OOTP itself as well as any OOTP data folders in any anti-virus/disk checker programs etc you may be using.
If doing that still doesn't solve this, then feel free to submit a
support ticket and the support team can walk you through some more troubleshooting.