This is to be seen as a starting point for troubleshooting and not the answer for everything (I will try though).

You read somewhere that it’s a bug

Sorry, it’s most likely NOT a bug.  If it stays after an factory reset and can be duplicated on other phones: then you MIGHT have a bug. But, it’s rarely a bug. Do a factory reset.

 You’re having problem connecting to your 7 year old wifi all-in-one router

Yes, even routers gets too old. Try changing encryption method and channel and so on. Try with a  different router (not the same type and age), if it works there: sorry, your router is past it’s prime. But, try THIS and if that doesn’t work:  factory reset. Sorted with the new 4.19.401.11 update.

You have 500 apps installed and everything crashes non-stop after the last update (before as well) and you can’t just understand why

Factory reset. At ONCE. Do not even get a coffee. Just: factory reset.

You battery lasts like a third of what it should (before update as well)

Look at the previous item: If your phone got that bad battery life, you have either some bad app installed (or several). Sure: if you play games 3 hours straight: you might need a charger with you. But: One with “normal” use should easy do a full day. If it doesn’t, you really should investigate just why it doesn’t. And, oh: Factory reset.

Yoda in a dream told you that “Reset to Factory you must”

Listen to Yoda. He got a point. Factory reset. Now.

In case my message isn’t clear enough. DO A FACTORY RESET!

If everything just fails, including a factory reset

Then you have more problems than just the update, contact HTC Support at once.

Want more info on this?

I’ve written about it a few times before, start HERE