Always debug with a non-browser client - much more helpful:
[m]
$ curl
http://mpkb.mp-dev.com/feed.php
<tr><td class="diff-blockheader" colspan="2">Line 131:</td>
<td class="diff-blockheader" colspan="2">Line 131:</td></tr>
<tr><td> </td><td class="diff-context"> </thead></td><td> </td><td class="diff-context"> </thead></td></tr>
<tr><td> </td><td class="diff-context"> <tbody></td><td> </td><td class="diff-context"> <tbody></td></tr>
<br />
<b>Fatal error</b>:
Allowed memory size of 67108864 bytes exhausted (tried to allocate 71 bytes) in <b>/home/jsmith/domains/mp-dev.com/public_html/mpkb/inc/DifferenceEngine.php</b> on line <b>905</b><br />
[/m]
Seems like there is some really huge change which causes the difference engine to exhaust the memory. You could either increase the allowed memory size or change the RSS type from diff to something other.