I looked at the original code for 1.7.10 as well as the code for our patched 1.7.10. There is a discrepancy in AntlrParserPlugin. The unpatched version can handle inner-interfaces, but the patched version cannot. It looks like this was missed during the latest upgrade of the patch.
Specifically, it looks like the innerInterfaceDef method exists in the unpatched version, but not the patched. Also, the objectBlock method has a case statement in its switch body for interfaces in the unpatched version, but not the patched.
I'm assigning this bug to Andy since he is the one who created the groovy compiler patch. The build server is currently out of service, so we can't get a fix out to you today, but we will once the server is running again.