The server response (from the browser console or apache error log) is
Error message for command: renderProblem faultcode: 404 faultstring: WebworkWebservice: Can't authenticate -- session may have timed out. Call stack The information below can help locate the source of the problem. o in WeBWorK::ContentGenerator::instructorXMLHandler::content called at line 232 of .../webwork2/lib/WeBWorK/ContentGenerator.pm o in WeBWorK::ContentGenerator::go called at line 384 of .../webwork2/lib/WeBWorK.pm Request information Method Post URI /webwork2/instructorXMLHandler UserAgent [user agent string]
Further confusing details: this fails with the same error if I revert back to WeBWorK 2.12. However, on a second server running WeBWorK 2.12 with (insofar as I can tell) the same operating system and configuration, rendering works fine. I turned on $UNIT_TESTS_ON in WebworkClient.pm and instructorXMLHandler.pm, and insofar as I can tell the same data are being sent back and forth on both servers.
There is one minor difference in that the install date for the 2.12 system that doesn't work was August 2016, while that which works is April 2017. But I think the configuration is the same.
I have tracked down the version numbers of the Perl modules that seemed most likely to be at issue, and they all appear to be the same between the two servers.
Any thoughts about what is eliciting the error are very welcome.