Whenever i post form over https I've got safari cannot open the page error. This typically happened when I had to log into a site. The same site would work find in Chrome however. This was really getting under my skin, but I did manage to find a solution and this is how to fix it. Please check attached image.
Go step by step and test.
1. Safari > Preferences > Extensions
Uncheck boxes beside “Enable Extension” and test.
Enable Extensions one by one and test.
To uninstall any extension, select it and click the “Uninstall” button.
2. Delete Caches
Close all windows and quit all applications.
Hold "option" key down and click "Go" menu in the Finder menu bar.
Select "Library" from the dropdown.
Library > Caches > com.apple.Safari
Right click the com.apple.Safari file and select "Move To Trash.
Close windows and relaunch Safari.
3. Delete com.apple.Safari.plist file and test.
Empty the Trash.
Quit all applications.
Hold the option key down and click the "Go" menu in the Finder menu bar.
Select "Library" from the drop down.
Library > Preferences > com.apple.Safari.plist
Right click on it and select "Move it to Trash".
Restart. Relaunch Safari.
If this doesn’t help, “Put Back” the com.apple.Safari.plist
Right click on the Trash icon in the Dock and select “Open”.
Right click on the com.apple.Safari.plist and select “Put Back”.
For more info: https://support.apple.com/en-us/HT204098
How to fix Safari Can't Open the Page error
