Hello Sindre,
First I use Subsonic and iSub-app iPhone based on Subsonic's engine & databaseserver for many years. The best music-server there is IMO ! Chapeaux1
I have read you have fixed the java-problem after saving changed user-settings with the version 5.1.beta2.
You told it happens only with Java 8.
See : http://forum.subsonic.org/forum/viewtopic.php?f=4&t=15086&start=15 somewhere in the middle

I use the WAR-version 5.2.beta1 for my QNAP-NAS TS-439 Pro II+, x86-based (Intel)
Java JRE v8.6 (2014/12/05 is installed on this machine.
Operating system Linux 3.4.6
Stil the problem is there : java.lang.String cannot be cast to java.lang.Integer
Exception java.lang.ClassCastException
Message java.lang.String cannot be cast to java.lang.Integer
Java version Oracle Corporation 1.8.0
Operating system Linux 3.4.6
Server jetty-6.1.x
Memory Used 69 of 96 MB
Stack trace java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer
at net.sourceforge.subsonic.dao.MusicFolderDao.setMusicFoldersForUser(MusicFolderDao.java:96)
at net.sourceforge.subsonic.service.SettingsService.setMusicFoldersForUser(SettingsService.java:1106)
at net.sourceforge.subsonic.controller.UserSettingsController.updateUser(UserSettingsController.java:159)
at net.sourceforge.subsonic.controller.UserSettingsController.doSubmitAction(UserSettingsController.java:113)
etc. etc.
Can you take a look at it?
Thanx in advance !
Greetz
Tom