By default the "Reclassify" box shown in Single Message View is empty (i.e. no bucket name is shown).
If the "Reclassify" button is clicked without selecting a bucket name then the "Single Message View" is replaced by the HISTORY page but this page no longer lists the message! This means the message cannot be viewed or reclassified.
The problem was introduced in POPFile 1.1.2 by changeset:3674 which changed /skins/default/view-page.thtml as follows:
r3453: <option selected="selected"></option>
r3674: <option selected="selected"> </option>
This means that the message was reclassified to a bucket called ' ' and the corpus ends up with an empty bucketid field for this message. This means the UI cannot be used to undo the reclassification.
The following Perl errors were generated when the "Reclassify" button was clicked without selecting a bucket name:
Use of uninitialized value in addition (+) at c:\temp\112\basic/Classifier/Bayes.pm line 547.
Use of uninitialized value in addition (+) at c:\temp\112\basic/Classifier/Bayes.pm line 561.
Use of uninitialized value in sprintf at c:\temp\112\basic/UI/HTML.pm line 2192.
It is possible that something else was broken by that changeset, e.g. at least one similar change was made to another file.