Mysterious Err:540 error code

I have a file with a bunch of formulas of all kinds, no recent changes.
Today, every single cell shows Error 540. It’s not even an actual LibreOffice error message, as far as I know.
Weird. I can’t find anything about it on Google, which is even weirder.
Any help would be appreciated!

Oh, right, it is! Thank you for that, Mike.
I had looked this page up:
https://help.libreoffice.org/Calc/Error_Codes_in_Calc

A follow up question, if I may. Usually, an “update” external files button shows up at the top of the page, but it isn’t there anymore. There also is a message in the status bar: External content disabled.
Perhaps a change of default setting after a software update?
I’ve tried changing security settings in preferences, as well as the linked files in Edit>Links to external files… but had no success.
Any suggestions?

  1. This is not a forum - this is an Ask site, where a question will receive one or several proposed solutions. Your reply isn’t a solution to your question; why have you used a box labeled “Propose your solution” to ask another question? You should either used a comment tool to add a comment to the answer, or - to file another question, to allow others looking for a similar thing, to find it (and answers to it).
  2. Unfortunately, I don’t know how that’s possible. I don’t remember such status bar message (do we really have a place for that in the bar?). You didn’t state your LO version; possibly a screenshot would help… and description what happened (an upgrade?); did you try resetting user profile, etc.

It is an actual LibreOffice error message; see relevant 6.3 help page for its description.

https://help.libreoffice.org/6.3/en-US/text/scalc/05/02140000.html

Err 540

External content disabled

Happens if a function that requires (re)loading of external sources is encountered and the user hasn’t confirmed reloading of external sources yet