Hello Razvan,
I would suggest debugging this in eclipse - it's very likely that you have two app store modules that have added different versions of the same jar. On build Mendix loads the jars ascending in order of name which leads to the older jars taking precedence in the build order.
If this is not the case you can still correct the error manually in eclipse.
You'll need to do the following:
1) Install eclipse
2) Open eclipse and navigate to File>Import
3) Select "Existing projects from workspace"
4) Navigate to your mendix app folder as your root directory
5) Import the project
Under your project you can now navigate to javasource and with a refresh there you should see clearly the origin of the error.
To find the libraries loaded navigate to Project>Properties>Java Build Path
Hope you find this helpful