How to solve this?
In apache logs we can find entries like this one:
[Wed Oct 15 20:51:05 2014] [apc-warning] Unable to allocate memory for pool. in /volume1/web/klasa/wordpress/wp-content/plugins/nextgen-gallery/pope/lib/class.component_registry.php on line 939.
In the php.ini increase memory reserved for APC.
vi /usr/syno/etc/php.ini [apc] ; APC Setting ; We now set "apc.enabled" in /usr/syno/etc/php/user-setting.ini ; apc.enabled = 1 apc.shm_size = 64 apc.shm_segments = 1 apc.ttl = 300 apc.user_ttl = 300 apc.num_files_hint = 1024 apc.mmap_file_mask = /var/spool/php/apc/apc.XXXXXX
In my case php uses 64 MB so i increased it up to 96 MB.
apc.shm_size = 96
and restart apache process (or reboot server).
If this will be still too low, increase and add more memory (like 128 MB).