Since no one from Mobisystems seems to want to answer here (why?) , I asked by e-mail and have now received an answer:
The employee wrote that he's "not familar how Tasker works".
And the AquaMail plugin for Tasker is "no longer supported".
I am very disappointed with this response. The plugin for Tasker is an important part of AquaMail for me. Please keep it alive, even if there will be issues in future.