Comment 3 for bug 306264

This is most likely caused by an incompatible change in curl-7.19.1, http://curl.haxx.se/changes.html mentions:
Fixed in 7.19.1 - November 5 2008 :
<snip>
 Bugfixes:
<snip>
    * recv() failures cause CURLE_RECV_ERROR

56 is CURLE_RECV_ERROR

It's a bit strange that pycurl-7.16.4 cause *more* failures instead of less (obviously *something* has changed in pycurl regarding that error handling... or should have change).

Regarding running some tests only (if you want to do a better diagnosis) you can do:

bzr selftest -s bt.test_http.TestWallServer.test_http_get -s bt.test_http.TestWallServer.test_http_has etc

to run only the tests that errors out (selftest doesn't display the leading bzrlib.tests that is abbreviated 'bt' above)

I don't have [py]curl-7.19 installed so I can't reproduce myself.