It should display the error "Update: Could not open update site #%d "%s", URL: %s" (with parsed variables) instead.
Don't know how you got the error but the solution is easy. The string has to be added to installation/language/en-GB/en-GB.ini and en-US/en-US.ini
Please test PR #26732
Don't know how you got the error but the solution is easy.
The string has to be added to installation/language/en-GB/en-GB.ini and en-US/en-US.ini