Cannot read from encrypted external SD Card - writing works though
Today a new update appeared in the phone, and another after that. It seems to work normally now. All apps can read and write to the external SD card. 

Glad I waited, because I was considering returning it to Sony to get this fixed. 

NB: The second update worked without unlocking the phone by the way. I don't like that! That happened on my previous phone as well. 

See Reboot encrypted phone without having to enter password and PIN after crash