I have had this issue out of the blue. MDT had been working flawlessly for around 2 years and then recently I kept getting to a point in a deployment when it would go no further.
When you are prompted for user, password and domain details so that the \\deploymentshare could be accessed I would either receive the errors:
Invalid credentials: The network path was not found
A Connection to the deployment share (\\computername\share\) could not be made. The
deployment will not proceed.
After a search on the Internet someone suggested changing the server name in the
bootstrap.ini from the ser ver name to include the FQDN i.e. \\mdt server\deploymentshare$ -> \\mdtserver.mycompany.com\deploymentshare$
This made no difference and I was beginning to think that this was a networking issue or port block issue of some kind.
Instead of using the FQDN I changed this to the IP address of the server i.e. \\18.104.22.168\\deploymentshare$
After updating the deployment share and then replacing the boot image used by WDS this resolved the problem.
I can only assume that there is some kind of authentication issue within Windows perhaps as a result of a Windows update