TrueZIP does a simple test to check if the archive file is writable. If this test fails, the archive file system is set read-only as indicated by the exception.
In most cases, this is simply an issue with the access permissions. But Windows is particularly bitchy. For example, if there is another tool concurrently accessing the archive file (many Explorer plug-ins do this) then the file is effectively read-only, too.
So please stay away from the archive file (and best, its directory) while the operation is running.