The error shows that you want to export the HMI files to the hmisrc directory. Can you tell me what is the content of the Output filename parameter in the REXYGEN HMI Configuration, tab HTML Export -> Expert mode? I assume this points to the location with no write permissions.

To clarify this matter:
When you are building your HMI inside the HMI Designer with using Export HMI (CTRL+H) extension, the Output filename parameter is used as the target path for both the HTML file and its resources.

When you are building the HMI using REXYGEN Studio with HMI block all the source files with suffix (*.hmi.svg) are built to the hmi directory using the same name as is the source filename.
E.g. file hmisrc/hmi-screen.hmi.svg is compiled to the hmi/hmi-screen.html file in the hmi directory.

Traceback (most recent call last):
  File "htmlBuilder.py", line 590, in <module>
    effect.affect(output=False)
  File "C:\Program Files\REX Controls\REXYGEN 2.50.8.10446\HmiDesigner\share\extensions\inkex.py", line 292, in affect
    self.effect()
  File "htmlBuilder.py", line 583, in effect
    saveFile = open(saveFile, 'w')
IOError: [Errno 2] No such file or directory: u'C:\\Users\\KrisS\\OneDrive\\REX\\REX_HEATING_RasPi\\hmisrc\\'

...but everything seems to be working fine afterwards.

It is telling that a certain directory doesn't exist but it does...It is located in my Onedrive folder so maybe some kind of permission problem?