Today, I recycled one of my host instances, and it didn’t start. Then I tried all of them, and several had issues. Finally, it dawned on me that I had changed the 64-bit config file:
“c:Program Files (x86)Microsoft BizTalk Server 2010BTSNTSvc64.exe.config”. The 32-bit host-instances were all starting fine, because it uses a different config file (BTSNTSvc.exe.config).

Check your web config file for malformed xml. (For example, make a copy called BTSNTSvc64.xml and open it in Internet Explorer to quickly find the error.)

I had something like this (to mailSettings close tags).

{!{code}!}czozMTE6XCIgIAo8c3lzdGVtLm5ldD4KICAgIDxtYWlsU2V0dGluZ3M+CiAgICAgIDxzbXRwIGZyb209XCJDbGllbnRTdXBwb3J0QHNve1smKiZdfW1lb25lLmNvbVwiPgogICAgICAgIDxuZXR3b3JrIGhvc3Q9XCJuZXdhZGRyZXNzLnNvbWV0aGluZy5jb21cIiBwYXNzd29yZD1cIlwiIHVzZXtbJiomXX1yTmFtZT1cIlwiIC8+CiAgICAgICAgPCEtLSBmb3JtZXJseSB3YXM6IDxuZXR3b3JrIGhvc3Q9XCIxMC4xLjEuMVwiIHBhc3N3b3JkPVwiXCIge1smKiZdfXVzZXJOYW1lPVwiXCIgLz4gLS0+IAogICAgICA8L3NtdHA+CiAgICA8L21haWxTZXR0aW5ncz4gICAgPC9tYWlsU2V0dGluZ3M+IAogIHtbJiomXX08L3N5c3RlbS5uZXQ+ClwiO3tbJiomXX0={!{/code}!}

That’s it, just fix your web.config hopefully.