The following minimal vimrc will trigger E474: Invalid argument:

setlocal iskeyword-=_
abbr <expr> _dts strftime('%a, %d %b %Y %H:%M:%S %z')

I've noticed this with editing ~/.config/git/config, which triggers setlocal iskeyword-=_ from /runtime/syntax/gitconfig.vim via vim-plug when explicitly loading a plugin marked for autoloading in the vimrc (this triggers BufRead and then the FileType autocommand after setf gitconfig). This can/should be fixed in vim-plug (by not triggering the BufRead autocommand before VimEnter), but I think the issue is still valid.

I think that the lhs being entirely made of characters from &iskeyword should not get validated when definiing abbreviations, but only (silently) when using them (i.e. they would not expand).