KeyError in external sources with "ListField" or "MultiListField" parameters after upgrading to Silva 2

Dear Silva Developers

I am upgrading my Silva instance from 1.4 to 2.04 and I found a major 
problem in handling External Sources with "ListField" or 
"MultiListField" parameters.

In my application I have an external Source "scs_projects" with a 
"MultiListField" parameter "locations" which holds some paths to documents.

In Silva 1.4 this MultiListField parameter is saved as:
   ['/path/to/document1']

In Silva 2.04 the parameter is saved as:
   /path/to/document1

Changing to the forms editor raises a key error:
   KeyError: u"['/path/to/document1]"

I am wondering how I can fix the described behavior...
Any idea?

I did a partial Silva content upgrade via
   services > service_extensions > Partial upgrades
and the log file returned me that everything was upgraded successfully.


Thanks in advance,
Nico