Well, after struggling with this error i think i found the problem...
I compared the joomla version om my working site (demo) with the non-working site (realworld), they are different. Demo site is 2.5.16 and realworld is 2.5.18.
I search the differences between those two versions and guess what...
"...Also with this release, the enhanced password hashing via PHPass, available in Joomla 3.2.1, is now used with the 2.5 series."
So, from 2.5.18 version, joomla changed the password mechanism, and i think that's what is causing all those login problems.
Can peolple check their joomla versions and post here?
Dimitrios, can you pleas check the mobile code to see if this is causing the login problem?
Regards