JIT traces not rendering

[rlamy]

Using a local vmprof-server instance, JIT traces can't be displayed any more after using the server for a while. At that point, all API calls (e.g. to "GET /api/jit/meta/7a53594c-f12b-439f-b5b2-62a6c63e1cef/ HTTP/1.1") seem to return a JSON containing just {"error": ""} and the spinner just keeps spinning with no user-visible indication of what's wrong. Digging down a bit, it appears that vmcache.cache.CacheProtocol._handle() fails with an AssertionError.