Custom EV3 Block: "Unable to compile..." ("string subsection")
A temporary fix I found that works: using the provided "gray blobs", there is one named "PBRStringAtIndex" (or something like that) which will indeed compile (EV3 only). This does not address the similar "write at index" issue, and the output is a "byte" (presumably ASCII), not text.

(I will update this answer as I find out more.)