I am experiencing a critical issue with my custom domains that has persisted for several days, and I have exhausted all possible troubleshooting steps on my end.

Affected Projects & Domains:

• Project: rtk-client-nojm | Custom Domain: telecom-rtk.ru | Vercel URL: rtk-client-nojm.vercel.app (works perfectly)
• Project: mts-r | Custom Domain: mts-s.ru | Vercel URL: mts-r.vercel.app (works perfectly)

Core Problem: Custom domains either fail to load entirely or load intermittently with ERR_CONNECTION_RESET 200 (OK) errors, while all corresponding .vercel.app domains work flawlessly.

Detailed Investigation Performed:

1. DNS Configuration:

   • Verified DNS records are correct across multiple providers (Timeweb)
   • Tried multiple A records (76.76.21.21, 76.76.21.22, 216.198.79.1, 216.198.79.2)
   • Confirmed global DNS propagation via DNS checker tools

2. Network Analysis:

   • Requests reach Vercel servers (Frankfurt region based on x-vercel-id: fra1)
   • Server responds with 200 OK and begins sending data
   • Connection resets mid-transfer with TCP RST packets
   • Issue occurs across different Russian ISPs (Rostelecom, MTS)

3. Architecture Changes:

   • Recently moved some backend APIs from Render to Vercel
   • Frontend makes requests to Vercel-based APIs
   • Confirmed no CORS issues (requests work via .vercel.app domains)

4. Geographic Testing:

   • Sites work reliably from other countries and most Russian cities
   • Intermittent success - sometimes loads after multiple refresh attempts

5. Headers Analysis:

   • x-vercel-cache: MISS consistently
   • cache-control: private, no-cache, no-store headers present
   • Successful TLS handshake, connection established then reset