Whenever I try to launch a modded version of Minecraft Java Edition (like Optifine) when using the latest version of the launcher (2.1.2337), it'll give me an error saying there was an internal error preparing that version.
I've seen 2 different errors so far, and those are HTTP 404: Not Found (with Optifine) and HTTP 403: Forbidden.
Vanilla versions (including 1.14 snapshots) don't seem to be affected.

EDIT: It looks like this bug has been fixed now in 2.1.2358.