Does not work on MSWin and Vim >= 8.2

[Grueslayer]

gives multiple errors, starting with creating the empty error file.

see vim/vim#8600

workaround could be to use :p:8 only on the existing directory and appending the new filename later (which is not 8.3 format and wasn't in the past, because VIM appended the given text).

[chrisbra]

can you create a PR with that?