UK user — had this after an automatic update pushed by Amazon. The Fix 1 (clear data) worked for me. Re-entered credentials, ticked the "remember" checkbox, closed and reopened the app. Credentials persisted. Has not happened again since.
Changing DNS from ISP default to 1.1.1.1 (Cloudflare) solved the no-signal issue for me without needing a VPN. The ISP DNS was redirecting certain stream hostnames.