Well, after struggling with it for a day now, I found out that I had the compiler level set to 1.7 in the library project, so when I exported the jar, it was exported with a higher compiling level than what android supports (1.6).
So the solution, in the project of the jar to export:
Properties
-> Java Compiler
-> Enable specific settings
-> Compiler compilance level
-> 1.6
That fixed it for me, I hope it's useful.