Hello,
Let me begin with saying Thank You to all of your help, Daniel and all others! I've searched these forums, read for hours and hours about everything from paths, MySQL issues, host related issues and all sorts of fixes for the Image Manager that comes with OC 1.5.5.1.
I've tried all sorts of stuff after reading your answers and advice, but my Image Manager is still not working. I have no more ideas to try, sorry. As you can see in the attached image, I get the same issues as many others do, and I have consulted my hosting as well, but without luck.
- I've set all paths correct in config.php
I've upgraded CKEditor to 4.1
I've explored the htaccess advises
I've tried deleting cache-folders
All folders have appropriate permissions, all according to the handbook.
My last attempt was not so successful either, I deleted all files and folders from the image folder. Then the image manager started working, I could create a folder from within the manager, and even upload a picture to that folder. But when entering the folder, the manager "crashes". Not crash, but reverts to the login screen issue.
I have no problem with manipulating code, as i am a web developer myself. But I can't seem to figure out this issue, it has been 5 days of searching up and down now. I've used Chrome and Firefox, and some other tools trying to pin it down. But with no luck unfortunately.
I really need help on this, I am happy to contribute in other areas. I could even consider paying someone to help me out.

I've developed an integration between OC and WordPress, and will release it later.