This is specifically for generic door type 18 Area Transition (No Visible Door).
In EE 1.80, if the door is placed in a normal doorway, it still works, but if the door is placed against a wall (or whatever) as a secret passage, it may work intermittently or fail altogether. Of course, the latter is rarely used, but, where it is, the module can be broken.
One fix is to move the door away from the wall by 0.1 m or so. However, that’s not really an option for older modules.
A better temporary solution is to reinstate tn_gdoor_08.mdl and .pwk from 1.69 in the EE override folder.
This works for all instances I’ve found so far.
P.S. Can anyone summarise for me the known door and secret door issues found in EE so far? Preferably with solutions, rather than just the module name. It might be worth developing a generic patch for older modules.