I once had Astrid installed, but I can't remember if it's on this phone or the previous one. But that shouldn't interfere, should it.
I had OpenTasks, that also presents itself with a icon named Tasks, but uninstalling it didn't help either.
Open up your device's settings, click on apps, and make sure Tasks.org is not there. If it is, tap on it, tap on the overflow menu, and select 'Uninstall for all users'
Then try downloading an APK from https://f-droid.org/en/packages/org.tasks/ and side loading it
You can follow up with me at support@tasks.org if you would prefer
I'll see if I can install the APK directly.