Can you try long pressing the attachment, choose Remove Download and then try to re-download again (just tap the attachment). If it still doesn't work let me know.
Unfortunately we can't really help you with your WebDAV provider. You have to talk to their support to figure out why you start getting 429 frequently.
I see you're using Koofr, maybe you'll find some help here - https://koofr.eu/help/koofr_with_webdav/i-am-getting-a-429-error-when-connecting-a-service-through-webdav-what-should-i-do/