Two possibilities come to mind:
- Dirty or faulty contacts: disassemble and clean them with high-grade alcohol (isopropyl if available) applied with cotton buds.
See iFixIt teardown:
FP2 iFixit Teardown
or a video recommended by AnotherElk (in German)
Fairphone Module - YouTube
or this in English: Replace a module for your Fairphone 2 | How to | Fairphone - YouTube - SD card.
do you have one?
if so, how is it formated? Either external / portable or internal. To find out, go to Settings > Storage.
Some important info on SD cards that everybody should read before using them with Android:
✏ A little guide to... using SD cards