Some iOS files contain invalid characters because the file name, for some reason is a date.

phpBB rejects these from being uploaded as invalid file names. Looks like there's code that checks for this here:

https://github.com/phpbb/phpbb/blob/master/phpBB/phpbb/files/upload.php#L266

However, this code is irrelevant because file names pass through clean_filename('unique') which means that it will be cleaned up anyway, so if you remove this code, nothing actually breaks and uploads with invalid file names works as expected.