It only happens in one site collection.
Soon I found the error message in ULS: "The requested workflow task content type was not found on the SPWeb"
Google search leads to this post. But re-creating Tasks list didn't resolve the problem in my case. The other posts all recommend to reactivate "OffWFCommon" feature. But it still didn't help.
Then I realized where the problem is.
SharePoint structure is based on "template". From the error message in ULS, it's easy to guess that the site content type "Tasks" is corrupted somewhere. Reactivating "OffWFCommon" feature only fixes the site content type template, not the content type instance in the Tasks list.
So the solution is simple.
1. Reactivate "OffWFCommon" feature
stsadm -o deactivatefeature -filename OffWFCommon\feature.xml -url http://sp.domain.local/sites/site1
stsadm -o activatefeature -filename OffWFCommon\feature.xml -url http://sp.domain.local/sites/site1
2. Rebuild "Tasks" list
Any comments welcome!