I am running firmware version 1.7 (b139) and have the same "Illegal characters found in URL" issue as described in this thread.
To answer Markus' question I am running the same version of the software that are listed.
If someone with 1.6 or 1.7 could check what versions they have for the following software:
device-client - 0.6-1
libcurl - 7.51.0-1
we may be able to track it down.