XQE-PLN-0098 The vendor specific function "Substring" is not supported

[dougp]

We have a report that has been working.  Let me emphasize that:  The report has been working correctly for months.  But today I started getting:

XQE-PLN-0098 The vendor specific function "Left" is not supported.

Inspecting the report, I find that it is using a data module that uses a couple of packages and a couple of uploaded spreadsheets.  It also defines a table based on stuff in one of the packages.

One query uses LEFT() against a SQL Server database.  That should work.
One query uses LEFT() against the table (based on SQL Server tables through a package).  That may work, but it may be abstracted a step too far and may confuse Cognos.
One query uses LEFT() against a spreadsheet.  I would expect that to NOT work.  Except Excel has a LEFT() function, so maybe it does.

So, when I started getting the error today, I figured the simple solution would be to replace

LEFT(str, length)

with

SUBSTRING(str, 1, length)

Since SUBSTRING() is in the functions list under Common Function | R-Z, this is a Cognos function, not a vendor specific function, right?

Now I get

XQE-PLN-0098 The vendor specific function "Substring" is not supported.

Ideas?
What could have randomly changed in my environment (no admin action taken) to bork this one report?
Why do I get this error message when Substring is not a vendor specific function?