Link to cell in another Calc sheet yields Err:540.

I am attempting to link to data in another spreadsheet by entering “=” in the refering cell and then left-clicking the referred cell in the other spreadsheet. It gives Err:540 in the referring cell. It appears not to depend on the content of the referred cell (text or numeric or link to another cell in the referred spreadsheet.)
LO V6.4.4.2 on kubuntu.

Save the new file, and reload it (choose menu File - Reload), or just close and reopen it. You will see an Enable Content button; press it.

See: Fixing error 540

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Fwiw, sounds like bug tdf#135108 but I never could reproduce it with a new document, only with a loaded document that so far had no linked externals.