Long story short: I kept thinking Activator was the problem since uninstalling it always meant an end to staring at the Apple logo. But after reinstalling it as the sole tweak and having no issues with soft or hard booting, I Googled and found this: [Question] Activator Bootloop, anyone else?
I don’t want to leave AdBlocker off my iPad mini 2. 😦 Guess I’ll do without Activator for the time being then.
Related to boot looping in general, this thread should be of interest to some: [Tutorial] How to avoid the bootloop (of death) after jailbreaking
Please be aware that, because iOS 9.1 is out, Apple will soon stop signing iOS 9.0.2 so if you want to jailbreak or re-jailbreak with the method outlined above, do it as soon as you can.
Edit: A friend with an iPhone 6+ says he has Activator and AdBlocker on his phone and has no boot issues. We even compared a list of other tweaks and apps we have like FolderEnhancer and iFile. .___. He also restored from a back up from iTunes after the jailbreak was done. Now I really wonder what the cause is.
Edit #2: I meant to update this a couple of days ago. ^^ Anyway, after turning some AdBlocker lists on and off, the same friend discovered that “The file com.yllier.blockerpreferences.plist in /var/mobile/Library/Preferences can’t be bigger than around 1.4MB or so”. He notes that the Privacy and Malicious lists are already that file size by themselves.
I’ve since disabled Malicious and the tweak works fine with Activator. No abrupt reboots or boot loops have occurred since.
I have also emailed the developer regarding the findings so hopefully, we’ll see some solution in the near future.