I would like to mention here about drop catching software for Dynadot's API - available for purchase at dropking.com.
This software works on any windows PC (32 or 64 bit) and allows you to instantly register expired domains on the fly. The software will connect to Dynadot's API & your domains will instantly appear in your account.
The software hits dynadot's API at about 1-2 tries per second & provides an instant log for your drop catching activity. The software provides domain registrations for all supported TLD's (domain extensions) that Dynadot supports. It's most popular for .com, .net, .info, .org etc..
You are correct, the expron software was upgraded to dropking.com which supports more registrars & has more features / software titles.
The expron.com software has always worked & continues to work. Perhaps you need to whitelist your IP address in your dynadot account so the software can connect to the API server. Many users use the expron software daily with success.
Please let me know if you need any assistance with the expron software. There is a free upgrade to the dropking software if you'd like to use that instead.