Example of this error:
Fatal error: Call to a member function templateFunctionName() on a non-object in /the/file/location/file.php on line 555
The error won't always be the same as above but you get the idea. Another symptom may be a blank/white page rather then the error. You can find bug reports on the IPS Bug Tracker here and here. As of IPB 3.2.3 this still appears to be a problem and goes back to IPB 3.0.x. Below is an IPS provided fix if you have this problem.
Manual Fix: (Provided by IPS)
Here's a work-around.
1) Make some skin OTHER than IP.Board the default skin, using the blue HTML icons in ACP -> Look & Feel.
2) Edit the settings for the IP.Board skin. The skin key field should show up, and it will likely be blank. Fill in the value: default
3) save the IP.Board skin.
4) Now use those HTML icons to make IP.Board the default if you want to, but you can leave it at whichever default skin you wish.
5) Go to ACP -> Look & Feel -> Template Tools -> Rebuild Master Skin Data -> select all the checkboxes in that section -> click the Rebuild button.
That will at least get the skin key set properly, which you can check in the ibf_skin_collections table of the database. The IP.Board skin should show the value 'default' in the 'set_key' field for the IP.Board skin entry. If it's there, that should resolve the problem going forward.
If you installed a new app shortly before fixing the underlying problem, and if it's not in heavy use yet, just uninstall and reinstall it. That should work, but if it doesn't for you, please file a support ticket.
As mentioned above, if new install then an uninstall and re-install with the provided instructions followed will work. But if you've upgraded or already have data added, please submit a support ticket with admin and ftp login details as the fix needs to be applied manually and is more complex.