whenever we change a function inside the RhinoCommon SDK, it can cause problem with Grasshopper on Rhino5. Grasshopper will be 'compiled against' the latest and greatest code, but while running on Rhino5 it will use the available RhinoCommon, which might be stale.
In any case, it will result in "Method not found" errors until Rhino5 has updated itself and the changed code becomes available again.
There should have been a Rhino5 update yesterday evening, can you try installing it?