Just to keep you posted. We have been in contact with the aolserver maintainer for debian for a while now. Similar incidences have been discussed over at the aolserver list, see e.g. http://article.gmane.org/gmane.comp.web.aolserver/15470.
To cut it short: the issue you encountered has to do with a fundamental conflict between the binary-level coupling of AOLServer core and its modules, on the one hand, and a packaging mechanism with dependency structuring as debian's, on the other hand. In situations of debian package *updates* (aolserver4-core, for instance), you risk running into situations that the module binaries delivered by the debian module packages (aolserver4-nscache etc.) are incompatible with the new core, causing faults of all kind. The inverse (old core, new module) might also occur but is less probable.
Anyways, the debian maintainer came up with a fix to guarantee binary compatibility of core and modules, starting with the upcoming 4.5.1, by playing an elegant trick on the debian packaging infrastructure.