What is your setup like?
(ie, Which PHP version? MySQL/MSSQL version?)
I found a previous thread in which similar error messages appear for a user using PHP 7.4:
Which I don’t believe is fully supported, as per the compatibility matrix:
In the same thread above, a user mentions modifying the error_reporting value in the php.ini
Does this resolve the issue?
If not, you could indeed delete the data, and re-import with the change you suggested
It’s not ideal that it ignores IDs, but if it works for now as a workaround, then at least you can get the data in the CRM!