This does only partially answer the question, but just notes where the error messages comes from:
Or name \@backslashchar\@qend... illegal,
see p.192 of the manual}\@eha}
The page number is coded into the error message, but does not specify which manual is meant actually.
texlive/doc/latex/base/manual.pdf we just find errata to the 2nd edition, which does not give any clue.
source2e.pdf, we find nothing really connected to this.
After using google, I found that it is fact the Book by
Leslie Lamport: LaTeX A Document preparation system.
(I found a link where a scanned copy of that book is available)
For reasons of copyright issues, I won't provide the link here.
Basic summary of the relevant page 192:
It's about the commands
effectively stating, that a command name must not be defined before to be used with
So if the error message occurs, the command has been defined before already.