Can't select database for transaction log backup in maintenance plan
See this Connect item, which is almost identical to the scenario you're trying -- they're close enough that I would say that it's highly likely to be the same issue. I would attempt to reproduce the exact same scenario presented in the bug report (I assume you will be able to).

While it's unclear in which release the fix for that bug landed, by the date of Microsoft's response, I believe this should be fixed in SP3.

If applying a service pack is not an option at this time, you'll want to go with a trusted 3rd-party solution, as mentioned in the other answer.