After installing a SharePoint 2016 Cumulative Update it is no longer possible to add the K2 Application to the Web Applications AppCatalog site. It was still unsuccessful after re-running the K2 AppDeployment.exe tool and trying to upload the application manually.
In this instance the issue was occurring because we did not fully patch the SharePoint environment correctly, there were still some update files missing which were in turn causing issues/errors.
Refer to this link for the January 2018 CU for SharePoint Server 2016.
Now pay attention to the part that says: "Important: It is required to install both fixes to fully patch a SharePoint server as each SharePoint installation comes with a language independent component and a language dependent component. If additional language packs are added later (only) the language dependent fix has to be applied again."
This means that to fully patch your SharePoint Server you will need to install both the language independent (KB4011642) and language dependent (KB4011645) fixes as per the link above.
Only one of the above updates were installed which in the end resulted in the issues presented.
After a discussion with Microsoft Support we were advised that the correct way to install the required patch is to follow these steps:
1. Install the first update (Language Independent)
2. Run the SP Config Wizard
3. Install the second update (Language Dependent)
4. Run the SP Config Wizard
5. Install a Language Pack (If any is required)
6. Run the SP Config Wizard.
Please note the SharePoint Farm Configuration Wizard needs to be run after every update.
After installing all the correct SharePoint files you will still not be able to add the K2 App to the EXISTING AppCatalog. You will need to delete the existing and create a NEW AppCatalog site for the Web Application at hand. Lastly you will need to re-run the K2 AppDeployment.exe tool to publish your App to the new AppCatalog and you should now be able to add the K2 Application successfully.