Just within this thread are 3 different customers stating they're seeing import errors. The topics in this forum topic are primarily import errors. There's even thread that seems to point to an import bug related to custom descriptions.
I even created a simple 1 step Write to File task in v10 right now, ran it successfully, exported to the server's Desktop, and tried to import that .aml into the v11 install on the same machine. Both x86, different install directories, different Task directories. No custom descriptions. Same error.
I'm nearly done manually rebuilding our tasks, and I'm certain there are no setup issues - as I've been running 4 environments for years (DEV, TEST, STAGE, PROD) and don't have any issues day to day. It's just the version to version import process that continues to plague many of us, and that doesn't seem to be addressed.