After an upgrade to Joomla 3.5.0 you receive "An error has occurred" on most pages in the configuration screens. This is due to a sort failing due to insufficient sort buffer size on the default synology /etc/my.cnf (which is linked to /volume1/@appstore/MariaDB/etc/mysql/my.cnf as the actual file)

Synology Default:

table_open_cache = 4
sort_buffer_size = 16K
read_buffer_size = 256K
read_rnd_buffer_size = 256K
net_buffer_length = 2K
thread_stack = 240K

To solve the issue you need to increase the sort buff by changing it to something bigger, but first you need to get root rights to do it and for this reason DO NOT attempt this if your not familiar with Linux. To get to root, login with any user in the administrators group, and then execute

sudo su

Enter the password of the admin account and your in as root. No please don't break it.

Now edit the file /etc/my.cnf (or the linked file as you please) and change the sort buffer to something like...

sort_buffer_size = 256K

And then restart the MariaDB server for the changes to take effect. You can do this via Packages, select MariaDB, then Action->Stop, then Action->Run.

Always check the /var/packages/MariaDB/target/mysql/servername.err log file (servername is the name of your server)

If your running a disk station with limited memory then you may want to crank that down until the error comes back to find the lowest memory allocation that works.

Finally, be aware that DSM Updates may overwrite this configuration file so if it suddenly returns after an update then you'll need to repeat the proceedure above.