Yes, debug was turned on. Below are some more lines from the error log.
DEBUG - 2021-02-22 08:54:32 --> UTF-8 Support Enabled
DEBUG - 2021-02-22 08:54:32 --> Global POST, GET and COOKIE data sanitized
DEBUG - 2021-02-22 08:54:32 --> Clients MX_Controller Initialized
DEBUG - 2021-02-22 08:54:32 --> Config file loaded: /home/vpsittard/domains/factuur.vpsittard.nl/public_html/application/config/invoice_plane.php
DEBUG - 2021-02-22 08:54:32 --> Encryption: Auto-configured driver 'openssl'.
DEBUG - 2021-02-22 08:54:32 --> File loaded: /home/vpsittard/domains/factuur.vpsittard.nl/public_html/application/modules/settings/models/Mdl_settings.php
DEBUG - 2021-02-22 08:54:32 --> File loaded: /home/vpsittard/domains/factuur.vpsittard.nl/public_html/application/modules/settings/models/Mdl_versions.php
DEBUG - 2021-02-22 08:54:32 --> File loaded: /home/vpsittard/domains/factuur.vpsittard.nl/public_html/application/controllers/../modules/layout/controllers/Layout.php
DEBUG - 2021-02-22 08:54:32 --> Layout MX_Controller Initialized
DEBUG - 2021-02-22 08:54:32 --> File loaded: /home/vpsittard/domains/factuur.vpsittard.nl/public_html/application/modules/clients/models/Mdl_clients.php
ERROR - 2021-02-22 08:54:32 --> Severity: error --> Exception: Unable to locate the model you have specified: Mdl_client_notes /home/vpsittard/domains/factuur.vpsittard.nl/public_html/vendor/codeigniter/framework/system/core/Loader.php 348
I can’t upload the full log file because it says I’m a new user 