My post text has gone white after 8.4 update

Since my wordpress update to 8.4 last week the text in my posts is white on the edit screen. The background is also white so the text appears to be invisible.

The text can cleary be seen on front end of the website where text appears as black.

My iphone can access the site and edit the posts as normal without any issues. My Mac is showing the text in white as described at the top of the post. If I highlight the text it appears on my Mac and a red line can be seen where there are spelling errors (the text above is invisible).

My blog has ground to a halt, please can you help me with this. I have deleted the cache and deactivated the plugins, I have changed to the default theme as well.