Just wanted to say that I am having SAME Problem, tried lots of fixes with adb and powered off cable and USB to Ethernet but it gives me no chance to allow debugging. I am going to click on above link for Fire TV Customer Support.
I had the same problem with my Fire TV 4K (The square one with the attached hdmi cable), but only if I had a USB hub connected to it. My solution was using a higher powered USB power supply for the hub. The Fire TV worked fine with a 1 Amp PSU, but went into the loop once I put the hub in the mix. Replacing the USB power supply with a 2 Amp unit solved my issue.
Hello, I had the same problem. I did not have my fire stick 4k connected to my laptop adb before I installed Google play so I could not uninstall play store and play services using adb. I came across this link and followed exactly what mentioned, It let me boot in to safe mode then I uninstalled play store and play services then did a normal boot. It works well now. (This url is non-English so allow google translate to English) I got y-otg cable from Amazon, I Kept holding ctrl+alt+menu+printscreen keys while another person assisted to plug in the fire stick power adapter. I kept hitting the "i" key. It started the regular white screen with orange amazon logo after few moments darkscreen+safemode in left bottom appeared then I got my screen. clicked "your applications" and uninstalled both culprits then power off and power on. (Both blue tooth and usb key board supports to invoke the safe mode)
This worked for me - Thank God. Take note that you have to quickly try to delete the files like every second for about 15 seconds and you'll catch the period in the machine's booting/re-booting that will allow you to properly delete them.
This worked for me as well. Do note that you have to quickly redo the adb command until it takes (it talks about this in the post, but I missed it the first time). It took me about 10 retries (up arrow, return).
I forgot I had been screwing around with Google Play Services the week before and could not get my 4K stick to get out of a bootloop. I'm a dev, so I already had adb installed. I had to find the right moment when adb server was running to get the commands to take. I've since done a factory reset and installed my apps. No more experimenting...