Merchant Security Updates: Learn how security upgrades may affect your integration at the 2016-2017 Merchant Security Roadmap Microsite
Payments Standard Updates: Beginning March 29th 2017, PayPal will offer a few enhancements that may require integration changes. Read more here: Payments Standard Upgrade FAQ
Can't remove existing applications, 500 http code in responses
Also can't view app credentials, see error - Something went wrong. Your sandbox app details could not be fetched. You can try again or you can fetch your live app details.
Can't create again sandbox accounts, which was removed, error - Email already exists.
And no responses from techsupport...
Tickets 170610-000049, 170608-000995
Unfortunately, this is all too common with PayPal's sandbox, especially REST. I'm still recommending people stick with the Classic APIs, and I still use them personally. Even though they are now labeled as "deprecated" they are not going anywhere for a long time. Still way too many people using them for a variety of reasons, including what you're running into here.