Go to command line, CD to where your dll file is located, and do

cacls yourDll.dll

You should get a list of groups and permissions, like so

C:\WINDOWS>cacls twain.dll
C:\WINDOWS\twain.dll BUILTIN\Users:R
                     BUILTIN\Power Users:R
                     BUILTIN\Administrators:F
                     NT AUTHORITY\SYSTEM:F

Verify that for your dll, every account that needs access, has it. If you're not sure what you're looking at, just post it here.