rdns dev here
MontBlanc Gone already again. I don't know why but somehow it does work since recently.
Check if you spot any relevant errors in About -> App Logs? You can use the search bar to search for domain / IPs that don't work.
Also, check Configure -> Network and Configure -> DNS for entries marked with red-coloured left-hand side border. These entries show DNS / connections blocked by Configure -> Firewall -> Universal firewall rules or per-app IP / domain rules (as set in Configure -> Apps, if any).
You can turn on Configure -> Settings -> Firewall Bubble which will create a small floating notification button that'll remain accessible without having to open Rethink. You'll see Firewall Bubble show all recently blocked apps, and will let one-click exempt those apps from ALL firewall / IP / domain rules for 15 minutes.
Make sure you haven't turned on any setting marked "experimental" in Configure -> DNS and Configure -> Network. Turning those on require you to know what its consequences might be and which to turn off depending on symptoms from when things don't work. Those settings aren't for everyone. Some of the popular LLM chatbots (like Kimi.ai, Z.ai, Gemini, ChatGPT etc) might help here (I give them a screenshot from Rethink and ask a question and I've found it to be surprisingly good at giving me good answers most of the time, but not always).
For more advanced debugging, Rethink's internal network engine state is also accessible from About -> General. It is rather a long wall of text, but you can copy paste it (to share it with me over email mz at celzero dot com; or with your favourite chatbot) and it might explain how Rethink has "setup" its network engine, currently.
Note that, logs in About -> App Logs and About -> Event Logs, and content in About -> General may contain sensitive information. So, do not share it as easily.