If you post offline while using a tablet then you cannot synchronise. (Except if you select a different post, although for 'Each person posts one discussion' that i snot possible.)

To reproduce it:

• Log in master using a student account
• Download the forum
• Go offline
• Go to the forum and write a post
• Go online and synchronise manually or wait. An error message will be displayed
  

Please note this only happens in tablets