X_DEV_LOCK_UNREADABLE
Means
the dev lock file cannot be read or removed
Typical cause
.x/dev.lock parses as no lock and unlink refused it — a directory left at that path, or a file this user may not delete. Its own code because the alternative was X_DEV_ALREADY_RUNNING naming the READER's own pid as the holder, whose fix: was kill <self>
Fix
rm .x/dev.lock, then re-run x dev