[OS X TeX] strange biblatex error - should I run mktexlsr?

[Murray Eisenberg]

After an interval of around 2 months, I am again processing a book-length memoir-class document under an up-to-date MacTeX/TeXLive 2020. The document uses biblatex and babel. Even though nowhere do I use basque, nonetheless I am getting now a totally unexpected error:
Package biblatex Info: Trying to load language 'english' -> 'mylanguagestrings'
...
Package biblatex Info: ... file 'mylanguagestrings.lbx' found.

(/Users/murray/Library/texmf/tex/latex/biblatex/lbx/mylanguagestrings.lbx
File: mylanguagestrings.lbx 
Package biblatex Info: Trying to load language 'english'...
Package biblatex Info: ... file 'english.lbx' found.

(/usr/local/texlive/2020/texmf-dist/tex/latex/biblatex/lbx/english.lbx
File: english.lbx 2020/08/23 v3.15a biblatex localization (PK/MW)
)
Package biblatex Info: Trying to load language 'english'...
Package biblatex Info: ... file 'english.lbx' found.

(/usr/local/texlive/2020/texmf-dist/tex/latex/biblatex/lbx/english.lbx
File: english.lbx 2020/08/23 v3.15a biblatex localization (PK/MW)

/usr/local/texlive/2020/texmf-dist/tex/latex/biblatex/lbx/english.lbx:571: Pack
age xkeyval Error: `langbasque' undefined in families `blx at lbx'.
In the source directory I have a biblatex.cfg that includes the line:

\DeclareLanguageMapping{english}{mylanguagestrings} % external file!

And in /Library/texmf/tex/latex/biblatex/lbx I have the referenced file mylanguagestrings.lbx which includes the following:

\InheritBibliographyExtras{english}

Unfortunately, I am unable to reproduce the error so far in a small document, just in the book-length document with its several long files input in the preamble.

Any suggestion on trouble-shooting this would be welcome!
---
Murray Eisenberg			murrayeisenberg at gmail.com
503 King Farm Blvd #101	
Rockville, MD 20850-6667	Mobile (413)-427-5334


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