Troubleshooting Hybrid Migration Endpoints in Classic and Modern Hybrid

YuriyGK - it looks like you've found your problem. As mentioned above, your server isn't able to be contacted using TLS 1.2.  You really need to update/fix whatever network device or computer is terminating your SSL/TLS sessions.  This will prepare you for the future when TLS 1.0 and TLS 1.1 are completely turned off in Exchange Online.  All that being said, they still should currently be enabled.  We're investigating some similar reports right now and are making a change to hopefully mitigate those reports.  However, officially Microsoft hasn't supported TLS 1.0 or TLS 1.1 since October 31 2018, so you need to get your environment supporting TLS 1.2 as soon as possible.  See this link for more information: https://docs.microsoft.com/en-us/microsoft-365/compliance/technical-reference-details-about-encryption