Hi Andrey,
A few questions if you don't mind:
- Are you aware of any changes that might have taken place recently?
- Does the issue occur across multiple browsers?
- Is Javascript enabled on your browser? If not, could you please enable it with this link (https://www.enable-javascript.com/) and try again?
- Is it 443 that the portal is listening on or have you configured a different port?
- Have you run a packet capture while trying to connect? If so, what were the results?
- Is the affected device an IP appliance by any chance? If so, it might be worth following sk39093.
I hope this helps.