Make sure the server is restarted after the update.
The fact that the error occurs only occasionally could mean that some of the apache server children handling the request have correct versions of the new module and some have versions of the old one. As the children die off and are replaced the behavior will skew toward the "newer" version of the module. If the children handle many requests before being killed off then this could take a long time.
WeBWorK Main Forum
Odd Occasional Error
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