Hello @Kacper_Kondraciuk
Feel free to check this video out on the link, Securely Self-Hosting n8n with Docker & Cloudflare Tunnel (The... Arguably Less-Painful Way) - #8 by King_Samuel_David
You could setup domain name, and https, which might solve this.
So to confirm also, your running n8n in dockers? on windows?
Where is your mcp trigger running inside n8n workflow?
Am trying to workout, as you mentioned in browser localhost works, so if n8n is in dockers that might be ure issue wrong hostname.
But id recommend to get a domain on it, your need it for certain apps because of callback URL etc.
Let me know am sure someone well answer 