Mail content not displaying on iMac after upgrade

Since upgrading to Tahoe 26.1 I have had problems with mail on my iMac. Mails arrive but some have no content - I just get the 'from' and 'subject'. I can access the content of these mails by looking at them on my phone so I know that they have actual content but can't access the content on the iMac. Other received mails work fine.

Any ideas ?