hi!
i had a same problem. in my situation users had to insert the token into the index.php?sid=xxxxx page and they did it by select->copy->paste and for several tokens people copied token with whitespace and pasted it. they were able to complete the survey but in limesurvey we could not see that.
i don't know if you have similar circumstanes but i think that might help. the solution is to trim token column in survey_xxxxx databease table.
this problem is rated as not reproduceble
bugs.limesurvey.org/view.php?id=02124