After a thorough investigation together with MultiLine we were able to pinpoint the cause and create a solution. We have updated the security components of Isabel 6 and MultiLine which have been published on our websites.
In order to solve the problem, please download and install the latest versions of both the components on your computer.
The installation of these components will solve the problem and you should be able to sign your transactions in MultiLine again.
If you want to test the signature with eID on a document, you can test with this document.