My 24-Sept-2013 comment for
http://webwork.maa.org/moodle/mod/forum/discuss.php?d=3108
described this bug (including the fact that it does not appear when an instructor is "Acting As a student").
You can view my contemporaneous Bugzilla report using URL
http://bugs.webwork.maa.org/show_bug.cgi?id=2806
My 7-Oct-2013 post at
http://webwork.maa.org/moodle/mod/forum/discuss.php?d=3128
describes what I have done as a crude substitute for a fix.
FWiW: merely clicking on "Check Answers" is sufficient to expose the link to Solution, entering an answer is not required. If the problem displays a "Get a new version" button, then clicking on it will also expose the link to the Solution.
OTOH: displaying another problem and then returning to the problem will require yet another "Check Answer" click before its solution can be displayed again.
WeBWorK Main Forum
Students can't view solutions (WeBWorK 2.7)
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day