I'm on the latest beta because thats part of the steps it says when you submit a problem, update to latest stable and then goto beta to see if it resolves your issue. I was original on 1.57, then went to 1.61 and now am on 1.62. Same experience on all 3. 1.57 formerly worked a few months ago, last time I tried it.
I named my dropbox share in rclone 'd', as you can see in the rclone.conf I pasted above that, so its not a drive letter.
I'm copying from the current working directory (which is what I use the . for) under /tmp/ads in dropbox.
I am running ubuntu 18.04 (I know this box really needs to be updated) but I have the same problem with the same config on 22.04 FWIW.
Can you try without the --ignore-existing? I wonder if an existing file is causing a conflict?
Note that the dropbox backend is sensitive to the leading / here d:/tmp/ads is different to d:tmp/ads. The /tmp one refers to a shared folder called tmp where as the other refers to a folder called tmp in your home.