The format of the timestamp has changed to ISO 8601. This contains the “:” Character.
A copy of the dll is saved at <home>/.openjfx/cache/" + jfxVersion + "/" + arch .
On Windows, the character ‘:’ is invalid in files, causing internal errors.

This only happens on developer/non-hudson builds, because on hudson-builds, the timestamp is omitted.

This bug was introduced in jfx21