When I update Drush to Drush 9.1.0, the following error now appears:
'drush' is not recognized as an internal or external command, operable program or batch file.
This issue has occurred on three different machines: two running Windows Server 2016 and one running Windows 10. I've read that it could be due to mismatched global and site Drush versions, but both versions are at 9.1.0. When I downgrade to Drush 8.x.x, it functions as normal. Both the global and site-specific Drush installs were installed with Composer. Are there any fixes for this error?