I know this works on the workbench:
SELECT Id,bizible2__BizibleId__c FROM Task
where bizible2__BizibleIid__c is on the activity table. This works too:
SELECT Id,bizible2__BizibleId__c,owner_manager__c FROM Task
but it doesn't work in simple_salesforce. No clue why. So I tried to simulate it because owner_manager__c is a calculated field that equals: Owner:User.Manager.FirstName &" " & Owner:User.Manager.LastName
the owner is a standard relationship for the task table and presumably the activity table. My attempt:
SELECT Id,bizible2__BizibleId__c,Owner.name FROM Task
works, but
SELECT Id,bizible2__BizibleId__c,Owner.Manager.FirstName FROM Task
Didn't work. Manager is a hierarchy thingy. I thought it could be because the owner relationship is user,calendar,something else and not just user, so I tried
SELECT Id,bizible2__BizibleId__c,LastModifiedBy.Manager.FirstName FROM Task
and that didn't work.
From what I remember "simple salesforce" uses REST and this API respects field level security. Are you sure you have access to the field? Just because you're sysadmin and bypass stuff in UI doesn't mean your Profile is all right. Workbench might be using SOAP API which is bit old and doesn't enforce the fields (yet).
What does this do for you?
On mutant fields sometimes you need to use polymorphic SOQL to get the fields you want. But still, the formula should work OK.