The line __ZN9FTimespan11FromSecondsEd traces back to an issue that was debugged out a couple of weeks ago. The root cause is reported to upstream Emscripten at Splitting double to 2xint32 parts in asm.js compatibility causes a wasm trap · Issue #5574 · emscripten-core/emscripten · GitHub.
To work around the issue, if one is building UE4 from source, removing -s OUTLINING_LIMIT in HTML5ToolChain.cs will fix the problem. I believe this fix should already be in trunk UE4 code, and be included in next release.