I am glad to hear that you have successfully resolved the issue.
In general, it is recommended that all data operations (paging, sorting, filtering, grouping, and aggregates) are configured to take place either on the server, or on the client. Mixing them might lead to undesired side-effects. In the current scenario where serverPaging is enabled and serverFiltering is disabled, the DataSource will filter only the current data page and you will see less results than expected. This is described in the DataSource's Mixed Data Mode Operations Documentation
In addition to the above, the minimal widget and DataSource configuration for enabling virtualization is shown in this section of the ComboBox Virtualization Article