I've installed a very basic custom module (contains one tiny model) and when i try to use it in a form i get the following error:
Sorry, you are not allowed to access this document. Please contact your system administrator if you think this is an error.
(Document model: {model name here}) - (Operation: read, User: 2)
While i'm logged in as admin and have developer mode on (should be irrelevant). I haven't altered anything in the security directory that was generated by odoo-bin scaffold ...
Since Odoo 12 the user OdooBot user (ID 1) is now the super user and doesn't need rights explicitly. But user Admin (ID 2) needs them now. I bet your tiny module does not have any right definition (ir.model.access.csv) in it.
So either implement rights or for testing purposes switch to OdooBot (developer mode context menu).