I just discovered that after writing changes of metadata to xmp sidecar files causes that in every affected album a subfolder appears which holds exactly the same pictures. The subfolder does not have a name. A closer look at the database revealed that for every picture in the original folder a second entry was written to the database. Deleting the subfolder deletes the whole album including the folder holding the pictures on the harddisk.

Reproducible: Always

Steps to Reproduce:
1. open digikam
2. change metadata (tag, title, etc.) of any image (it do not have to be all, one is sufficient) in album
3. save changes and write them to sidecar file (in my case nef aka nikon raw files)

Actual Results:  
Subfolder appears as described. At the same time for all pictures in the album a second entry is written to the database.

Expected Results:  
No subfolder appears and the old entries in database are changed instead of being copied.

On my system 5.0.0_beta3-1 is installed. But that did not change for a while and it worked with this version until recently. If you let me know which other components are involved I could look for recent updates of these.