Thank you for your post.
If I could confirm and gather more information:
-Your two client machines are running on Widows 7 Ultimate SP1?
-Have you compared the applications and services running on these two machines VS the "working" machines?
-Is this issue happening when working with a local database? (not connected to the FileMaker Server)
-What are the clients doing when the crash occurs? (send mail script, performing a find, entering data into a field, etc)
It seems you are using Remote Desktop Services on Windows Server 2012 with a Volume License of FileMaker Pro 14.0.5, which is supported.
-Just to confirm, what environment do you have FileMaker Server installed on? (Windows Server 2012 R2 Standard, etc.)
-What version of FileMaker Server are you running? (FileMaker Server 188.8.131.524, etc.)
Also, keep in mind Page 9 of the Remote Desktop Guide. I can't tell from your description if this may be the case.
Note: FileMaker Server should not be installed on the same machine with Citrix XenApp and Remote Desktop Services. FileMaker, Inc. does not support FileMaker Server running on the same machine with Citrix XenApp and Remote Desktop Services.
I look forward to your response!