Another reason can be that your PB app is compiled as a 32-bit EXEs and the MS Office products installed are 64-bit EXEs.  There may be some scenarios where the bitness does not need to match, but in most cases the bitness does need to match.  We this problem often... for example using 64-bit DB driver with 32-bit EXE... or as another example, customers that compile their PB project to 64-bit but still left external DLLs, ActiveX, etc. as 32-bit versions.  In general you can't mix n' match.

Are they using DDE to talk to each other or with Outlook?

Microsoft disabled DDE in all Office products as of the December 2017 security update. It could be that Outlook is blocking all DDE occurring on the PC.

To communicate between two PB apps, consider using Winsock:

http://www.topwizprogramming.com/freecode_winsock.html