I found something regarding the host browsing. Once a file with blocked URN (malware) is in the browsing result the connection gets aborted.
I changed this in SVN to just skip the bad file.

I had no problems with queuing though...
To help debug this better try to add the following line to your phexCorePrefs.properties:

Download.CandidateLogBufferSize = 1000000

This gives about 1MB of extra memory for holding a candidate download log. You will see a Log button on the candidate toolbar. Pressing it shows the log of the candidate. The log shows the exchanged dialog and also includes status changes and errors. This log might give us an idea where and why something could go wrong.

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