Cause of Cannot execute as the server principal because the principal "MyUser" does not exist'

Question

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 [edit: and are not orphaned - just double-checked]. 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:

name	type	usage
MyUser	SQL LOGIN	DENY ONLY
public	SERVER ROLE	DENY ONLY

And then from sys.user_token, on DB1:

name	type	usage
MyUser	SQL USER	GRANT OR DENY
MyRole	ROLE	GRANT OR DENY

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

name	type	usage
MyUser	SQL LOGIN	GRANT OR DENY
public	SERVER ROLE	GRANT OR DENY
sa	SQL LOGIN	AUTHENTICATOR

And finally from sys.user_token, on DB2:

name	type	usage
MyUser	SQL USER	GRANT OR DENY
MyRole	ROLE	GRANT OR DENY
dbo	SQL USER	AUTHENTICATOR
dbo_owner	ROLE	AUTHENTICATOR

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? (As this is in a highly accessed PROD environment I don't want to make any unnecessary changes)

Any help much appreciated!