I have several spreadsheets linked to a main one. Today when updating the
source data, I get the error message "formula too long". (The new source
document link has exactly the same number of characters as before.)

This has never happened before - there are not 1024 characters in the link
so I do not understand why it has happened.

I have searched this and the error user groups but cannot find anything that
relates to this error message being associated with links.

You assistance is greatly appreciated.