This probably happens due to the multiple bootstrap files being loaded on the frontend site. PEP enqueues bootstrap files on the frontend as well as on the backend. The issue is generated when PEP and theme (or another plugin) enqueue the bootstrap on the frontend.
To fix the issue, enable the setting ‘Disable Bootstrap on frontend’ to disable the PEP bootstrap files. This setting can be found under: Product Enquiry Pro menu > Settings sub menu > Display tab > Disable Bootstrap on frontend setting].
On the backend, bootstrap files are loaded on the custom form page and ‘Create New Quote’ page. So, bootstrap files will still be loaded on these backend pages.