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
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