I have two databases on the same server, lets call them DB1 and DB2. There is a login on the server called MyUser, and both databases have a user setup from that login, also called MyUser. The following works fine in DB2, but not in DB1:

execute as user =  'MyUser'
execute as login = 'MyUser'

When I run this in DB1, I get:

Cannot execute as the server principal because the principal "MyUser" does not exist, this type of principal cannot be impersonated, or you do not have permission.

As far as I can tell both MyUser users belong to the same roles on their respective databases and those roles have the same permissions that they grant.

To try and figure this out I've run the first execute as and then selecting some further information as follows, and this is where it gets interesting:

execute as user =  'MyUser'

SELECT *
FROM sys.login_token
WHERE principal_id > 0;

SELECT *
FROM sys.user_token
WHERE principal_id > 0;

Now, the output I get for these is as follows. For DB1 (the problem database) I get (leaving off the pricipal_id and sid cols), firstly from sys.login_token:

And then from sys.user_token, on DB1:

Now, running the same thing in DB2, firstly from sys.login_token:

And finally from sys.user_token, on DB2:

I can't see anything different between the dbo database User and dbo_owner Role objects between the two databases, nor how the sa server login applies at all differently to the two databases, and have no idea how they contribute to these "tokens".

Where should/could I look next to solve this puzzle?

Any help much appreciated!