Hello.
I run the site: edisal.salesianos.pt
Customers when they try to choose (or change) the payment method receive the warning "Error updating cart".
The problem seems to happen with all payment methods (and not only with paypal).
When I go to VM configuration and turn off "Ajax for OPC", this error disappears; customers can choose their payment method.
But, then they go to a warning page: "Cannot write to log file."
Everything was ok yesterday; there was a lack of disk space, that blocked the system; after the increase in disk space this error started.
My configs
Joomla 3.10.2
Vm: 3.8.8 10472
PHP: 7.4.14
DB: Mysql 5.5.5-10.2.36-MariaDB-10.2.36+maria~stretch
log_path is properly set (755/644)
tmp_path is properly set (755/644)
If the error did not occur before something needed to change at their end then it seems logical that you might be better talking further with you server provider.