I have installed the se-sv language Pack 4.8.2 and tryed it with en-us, also as with the swedish language. No issue found out.

Please check your existing web.config file against a new web.config file of the same version of your DNN.  As I heared there was a issue on auto upgrading the web.config file on update to DNN5 .  Not sure if this is the issue, but it could be a point. 

Have you check the install log after upgrading to DNN 5.1 ?  Was there any issues caused?

I have now also test it with DNN 5.1.0 and the se-sv langauge pack, and could not reproduce your issue.  But I notice it is a fresh install of DNN 5.1.0 about my existing dnn 4.9.4 environmont is needed at time for creating new language packs.

This tooks that you had a issue on a previous upgrade. 

Jan, I would run this: SELECT  * FROM ModuleSettings WHERE SettingName = 'LastIndexDate'

It should return a row that has your ModuleID and a SettingValue which should be something like this: 40021.7910372801. If it is not in that format, simply replace it with what I have and you should then be able to load the page.