V7.4 to 7.5 Migration Issue - White screen after login

Trying to migrate V7.4 on Ubuntu 20.4 (php 7.4) to V7.5 on Mint 21 (php 8.1) and, following migration then moving from Ubuntu to Mint, I get a white screen after login. If I enable error reporting, I get the following message: Fatal error: Declaration of EmailTemplates_Module_Model::saveRecord(EmailTemplates_Record_Model $recordModel) must be compatible with Vtiger_Module_Model::saveRecord(Vtiger_Record_Model $recordModel) in /var/www/html/vtigercrm/modules/EmailTemplates/models/Module.php on line 35 Anyone got any advice on this please? I've tried moving the vtiger folder and database to Mint then running the 7.4 - 7.5 migration patch (i.e. under php 8.1) and also doing the migration patch before moving the folder and database (i.e. under php 7.4) with the same results. A clean install of vtiger 7.5 works fine on Mint 21. Anyone else experienced this issue?