Symptoms
This hotfix should allow the file upload if files are larger than 5 MB.
The DLLs were applied on the PROD environment as described in the readme.txt, but upload of the large files is still not possible (see attached screenshot).
Diagnoses
We narrowed the search down to permissions that the K2 service account is to be admin of sub site collection.
Resolution
Setting K2 service account as admin on the subsite collection resolved the issue from K2's side.
The client will look into this matter from Microsoft's side.