hi,
you talk about "com dll", if you declare a .net variable that directly runs a "com object" and not a dll object > "COM is no longer supported on the Microsoft Dynamics NAV 2013 Server tier (only on 32bit windows client)"
in other cases, you you talk about "dll" may be a problem with 32 bit\64 bit dll registration (or compiling tipology); whe you run from nav windows client, system check 32bit dll, on server tier (and nas port) sytem check 64 bit dll; you need to register dll on 64
look at this post about dll registratrion