Bookstack is a foss wiki system
Image Gallery Running out of memory
The image gallery can run out of RAM whilst generating thumbnails when you select images from the gallery. By default nginx only provides 128MB of RAM for each PHP thread so increasing the limit can help. See this github thread
If using the LinuxServer docker-compose solution, find and edit config/php/php-local.ini and add a line like the follownig:
memory_limit = 512M