
I just met the same problem as yours almost two weeks ago. I even tried for several ten sites to test which one is good for limesurvery work well. But the results is always same, I can't login as admin, and always it came back the login webpage. Just when I want to find other method whether I don't know it would be ok, it is really made me head pained, some sound reminded me why not try the easiest possibility, last year when I first time tested limesurvey, it sound the config.php must be saving as utf-8, just like the special explanation in installation profile for limesurvey.
I should short my words: I found out the software named PSPad editor in my Win PC, edit the config.php for my site parameters, and saved it. Then transferred the file to the server, open the admin webpage. It worked now!
Finally I understood when original time I install limesurvey to the sites, I am lazy, just used the Notpad of Win to save the config.php, but in fact, Notpad changed the file format silentally although it said would be save the file to utf-8. That is the true reason why I can't login as admin, just like what mentioned in the installation profile of limesurvey.
So, at first, try the PSPad editor to keep your config.php as true utf-8 format, then test others.
Now almost all of my tested sites can smoothly running limesurvey, that's true limesurvey performance.
That is my funny story when I use the very nice software limesurvey, it sounds just like the story about tailor fighting with the huge human-being through the magic bean, hahaha!
Thanks for your time to read my words, and hope these is useful for your.
Good luck! always!