I just upgraded from allegro_free_viewer_16-5 to allegro_free_viewer_16-6. Right after the upgrade it worked fine but today I keep getting the message
This application has failed to start because callbackMgr.dll was not found. Re-installing the application may fix this problem.
I uninstalled and reinstalled the viewer but same results.
I did a regsvr32 "C:\Program Files\Cadence Design Systems\Allegro Free Physical Viewers 16.6\tools\pcb\bin\callbackMgr.dll" to register the dll but I get "Dll_Name was loaded, but the DllRegisterServer or DllUnregisterServer entry point was not found."
I had the same problem, and solution was to copy file "callbackMgr.dll" from:
C:\Program Files (x86)\Cadence Design Systems\Allegro Free Physical Viewers 16.6\tools\pcb\bin
This solution if for 64 bits systems. For 32 bits, solution is in the last answer.