1. Error: DTD is prohibited in this XML document
When you try to login to your SharePoint.Online might happen that you get error message "For security reasons DTD is prohibited in this XML document. To enable DTD processing set the ProhibitDtd property on XmlReaderSettings to false and pass the settings into XmlReader.Create method."
Root cause has something to do with DNS and network card configuration.
From article https://melcher.dev/2016/02/for-security-reasons-dtd-is-prohibited-in-this-xml-document/ solution was to disable IPv6 and to set DNS server to 22.214.171.124 and 126.96.36.199.
With one of our customers we tried this approach and after applying this change all worked ok.
2. Error: LegacyAuthMode disabled
When you try to connect to your SharePoint site (mainly SharePoint Online) you see error similar to:
(key is: X-MSDAVEXT_Error=917656. Access+denied.+Before+opening+files+in...)
This error message points that LegacyAuthMode is disabled (set to false). To go around this one solution is to use App-Only mode. How to configure App-Only mode can be found here.
IMPORTANT: if you received any other error message please let us know. Email to firstname.lastname@example.org with error screenshot will be appreciated.