So that this doesn't cost you an hour of Googling around:
Problem: launching psexec (from Powershell) with ./psexec64 \targetip -s -u domain\username -p "password" c:\path\to\this\file.exe -accepteula
Result: It seems to connect but you get All Pipe Instances Are Busy
The solution (at least in this case: The path to the executable on the target host has a typo.
Yup. No shame. We all do it. Go fix your typo and enjoy the rest of your day.
On a related note, I couldn't shake the "the handle is invalid" error from the command prompt and worked around it by going to PowerShell. Apologies for not having a good technical explanation as to why that's the case, but this has been bonus psexec material.