I only need tasks in one of my GoogleApp domains, and that just happened to be the one where I was receiving the error "tasks could not be open" after enabling the feature.  What?  Why?  This is stupid.  Thankfully it didn’t take long to realize the one difference between that domain and the others I manage was the disabled chat feature.  Apparently Tasks uses the same API as Chat for making the in-window window and the pop-out window.  (Who knows what else.)

Of course now I have chat enabled somewhere a client doesn’t want or need it.  Not sure if this issue will resolve itself in the future or if we’re stuck with both features having to be enabled.  In the meanwhile, if you or someone you know are having trouble getting tasks to work in Gmail, hope this helps.

Update: Seems this issue was added to the Google Labs Help Tasks group discussion earlier today, and a fix is in the works.  You really have to enjoy the process.

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Google Labs Adds Gmail Tasks, Requires Chat Enabled was first posted to justinll.com on December 9, 2008.