Falls es jemanden auch interessiert: Die PACS Einrichtung auf Synology erfolgt so:
1) Install Java7, then MariaDB. NOT by selecting the PACS package and getting Java7/MariaDB as dependencies... install them individually. Don't get near the PACS package yet.
2) Launch MariaDB and configure it before installing PACS. Select the volume you want your databases to live on (and click 'Apply'), then click 'Change MariaDB password' to configure a root password. By default the root password is blank, so when it asks you for your password on the next 'Log in to MariaDB' window, just click OK. Then in the 'Change MariaDB' dialog box, the username is 'root' (should be the only option in that drop-down selector), and enter a new password in both blanks. Maybe the PACS installation can still work if you leave the MariaDB password blank, but that wasn't working very well for me, hence the decision to install MariaDB separately and configure a root password before installing PACS. In my case the MariaDB password is the same as the admin/root password for my whole Synology system, but I am PRETTY sure it doesn't have to be that way... I think you can pick anything you want for the MariaDB root password. Click OK to confirm and get out of MariaDB.
3) Now install the PACS package from the Synology package center. It will ask you to provide a database name (I just used 'pacs') and your MariaDB root password, which should be the new MariaDB root password you just created in step 2. Then you will get to enter a PACS username and password. I THINK this can be anything you like, but I used the same username/password combo I use to log into the Synology, just to be on the safe side (e.g. username 'mattrjohnson', password 'hunter2').
This should let installation finish successfully, and then you should be able to log into the PACS pages (the PACS-manager dcm4chee page, the Oviyam viewer, and the PACS-Settings JBoss page) with the username/password you defined at the end of step 3. From there you can go on to configure the system, set up additional users, etc.