Hi Alfred,
I would like to know the exact error message when you sign in, in the meanwhile please try the following steps assuming its Windows, let me know if its a MAC:
1) Go to Start button>> Control panel>> Uninstall a program.
2) Check if you see Browser safeguard there, if yes then uninstall it.
3) Open Internet explorer>> Tools>> Internet options>> Connections>> Lan Settings>> and it should be like this.
-Ankit