If the path of the liberkey root contains blanks, such as:

f:/my portable stuff/Liberkey/

... no application can be launched, although updating works fine. Move the Liberkey dir to another dir without blanks:

f:/my_portable_stuff/Liberkey/

... then everything works fine.

A bit annoying.

Update:

Liberkey path with blanks is apparently not the cause of the problem, since today the error is no longer reproducible.