Type of zcrmsdk\crm\exception\ZCRMException::$file must be string (as in class Exception)

Type of zcrmsdk\crm\exception\ZCRMException::$file must be string (as in class Exception)

Upgrading an app from PHP 7.4 to 8.1, using the same zcrm_oauthtokens.txt file and 
zohocrm/php-sdk-archive   2.2.1

I get the following exception
Type of zcrmsdk\crm\exception\ZCRMException::$file must be string (as in class Exception)

Any idea what causes this?
TIA
Dave