Mail on High Sierra not showing content

I have all my accounts IMAP

On opening Mail Version 11.5 (3445.9.5) on High Sierra (10.13.6), there's no content showing. The headers are all there, and I can view the raw data. but the body shows as blank, at a certain point after restarting mail, the content of a mail I clicked on, showed for a second and then disappeared.

On opening it in Mail on my other computer, iPad and iPhone, the content shows.

This is an old iMac 2011, which I just put on a SSD new disk and it's flying, but I just found out this problem as soon as I open mail.

What's happening?