my calendar app is going through the exact same thing (no numbers / faint writing) and I've had to download another calendar app to go about day to day for now, did they advise you do something manually by any chance or would I have to call them myself?
Glad yours is fixed
When I rang i at to download this app which allowed them to take over my phone and sort it out, I didn't take to much notice off what they were doing so apologies for that I cant help you there,it didn't take them long at all
I had the same issue : white letters on a white background.
It was because of my theme. I was able to find an update for my theme, and the issue has been fixed ! So if you are lucky (and you remember the name of your theme) you can also find an update for your theme. Then you will not need to choose another calendar app/ agenda app.
If there is no update available for your theme (yet) you can choose another theme.