Good, you found out, what was causing the trouble! 
My suggestion: Upgrade to the Unified Storage Partition, which can only be done from 1.6, and wait until the revised version of 1.8 is released. There were some problems with 1.8 so Fairphone decided to postpone Fairphone OS 1.8.
If you are having further questions, please search the forum or open a separate thread. I will also update the title of this topic.