UPS Shipping Plugin Causing Critical Issues

I have tried deactivating each plugin in turn and discovered that the UPS Shipping plugin is causing 3 critical issues on my client’s site. The issues are:

1. An active PHP session was detected:
A PHP session was created by a session_start() function call. This interferes with REST API and loopback requests. The session should be closed by session_write_close() before making any HTTP requests.

2. The Rest API encountered an error:
The REST API is one way WordPress, and other applications, communicate with the server. One example is the block editor screen, which relies on this to display, and save, your posts and pages.

The REST API request failed due to an error.
Error: cURL error 28: Operation timed out after 10001 milliseconds with 0 bytes received (http_request_failed)

3. Your site could not complete a loopback request:
Loopback requests are used to run scheduled events, and are also used by the built-in editors for themes and plugins to verify code stability.

The loopback request to your site failed, this means features relying on them are not currently working as expected.
Error: cURL error 28: Operation timed out after 10000 milliseconds with 0 bytes received (http_request_failed)

Any help appreciated! Do let me know what further information you need.

Thank you in advance.