Problem: Access Denied for the 'super' admin and later, same for the non-super admin.
After Masquerade was installed, it was tested with an Administrator (not UID 1) on a PC for a test user in a newly-created custom role. It seemed to work fine.
A short time later, when the 'super' admin (UID 1) tested it on a different computer (Mac), 'Access Denied' was the result, even though all 3 module permissions were checked for the admin role and the custom test role had only the 'Masquerade as user' perm checked; and the Masquerade config via Structure -> Block had both the Administrator and the custom role checked.
The 'non-UID 1' admin (PC) was still able to switch in both directions, although the 'Switch Back' from's username was blank on that page (there was a space between the last word and the period). [*** NEEDS FIX ***]
To further check, a 2nd test user was created to be sure that it was not a permissions problem that may have developed after a 2nd test role was created, whereby the 1st test user was moved to that 2nd custom role after using Masquerade initially. The 'non-UID 1' administrator (PC) was still able to switch back and forth when UID 1 (Mac) could not.
Note: The block's role permission was updated to reflect the test users being assigned to the new 2nd custom role (one had been moved and the 2nd was assigned there when user was created). Administrator role was already checked.
Upon logging in as the 'super' user for further troubleshooting, only this time on the PC, 'Access Denied' for the 'super' user was still the result for both test users that were in the same role.
The bigger problem now is, once the 'super' user was logged in on the PC and trying to test Masquerade which still showed 'Access Denied', now the 'non-UID 1' admin is showing 'Access Denied' for both users, whereby neither admin can use the module.
EDIT: btw - domain's cookies were deleted and cache cleared on the PC between logging in as UID 1 and non-UID 1.