Behavior:
If you try to send documents directly from the WebClient you receive the MAPI Error 26.
Answer:
Installing Update KB4011178 for Microsoft Office can cause the issue described above. In order to fix it, you need to repair Microsoft Office Outlook:
https://support.office.com/en-us/article/Repair-an-Office-application-7821d4b6-7c1d-4205-aa0e-a6b40c5bb88b
After that it should be possible to send documents from WebClient again.
