Do other OAuth-enabled connectors work (Google Drive, Gmail)?
One case can be the windows firewall blocking inbound connections. You can temporarily (!) disable Windows Firewall to check if this is the case (go to Windows Security → Firewall & Network protection → click active network name (it should be either private/public/domain) → set Windows defender firewall to OFF)
If that helps, you can add a manual allow rule to the windows firewall, or (sometimes that doesn’t help when specific block rules take precedent over allow rules) pinpoint the exact blocking rule that causes the problem.
To do this, type wf.msc in Windows start prompt to launch Windows Defender Firewall with Advanced Security.
Then, to add allow inbound rule right-click Inbound rules → New rule → pick Port → Next → pick TCP, and in Specific local ports write 7890 → Next → Allow the connection → Next
If that wouldn’t help, you can search for a blocking rule. In the same Windows Defender Firewall with Advanced Security window click Inbound rules and click Action column header to sort block rules to the top of the window. There you can see all existing and active block rules and you can try disabling them one by one to find out who blocks the Easymorph OAuth process.
The third option is to reset Windows Firewall to a pristine out-of-the-box state. This option makes sense only if you already found out that disabling windows firewall solves the problem but for some reason can’t find the exact rule set that causes problems. To do this, you can right-click Windows Start button, pick Windows Powershell (Admin) option and in admin console type
netsh advfirewall reset
command