You need to check the forum permissions.
There are settings based on what type of user on what forum and how many images and how big a image.
Within the limits of your php.ini settings.
Goto ubb cp.
Select the forum in question at "Currently Editing Permissions For" and then select switch
open the "Max gallery images " edit it to how many images allowed per post by member type.
Then open "Max gallery image size" edit it in bytes per image allowed per post per member type.(there is a note with this setting to give you a idea for size.)
When done at the bottom select "Update Permissions for selected Forum"
when adding a attachment to a gallery forum there should be a note that tells you how many you can post plus the max size per attachment.
There is another php.ini setting called post_max_size
Ii is a size limit for the sum of all the attachments in one posting that you should check also.
Last edited by Ruben; 11/06/2019 4:44 PM. Reason: Added PS