Sorry, this is not very helpful, nor can I reproduce it anymore, but:
Beta release 2 was not working on my iPad 3 (wrong ARM compliation issue), but I needed Pleco, so I nuked Pleco and reinstalled from scratch.
Every time I tried to use "send to" from Safari with an epub, or tried to access my dropbox folder from within Pleco, Pleco would crash.
I then tried with The "beta" (ie. non-release) version of Beta Release 3, and was seeing the same issue.
I don't see it anymore with Release 4, but I also had tried loading a PDF first, which (possibly) seemed to make things go smoothly after.
I checked the ~Library/Logs/CrashReporter/MobileDevice/* for crash logs after syncing, but nothing showed up for Pleco
