Thank you all so much for helping me!
I've followed the steps of the pdf regarding the "usb burning tool" but it cannot find the U22-XJ!
I don't know what i'm doing wrong!
PS1.: Except from the "FOTA update" app there is also an app called "update" which seems it can load files from "local" directory! I've tried 2 usb sticks with the "20200826" image file but while these sticks are recognisable - by both the ES file explorer and the preinstalled file explorer - they appear as "empty"!!!
PS2.: Lately i've noticed Youtube is also "freezing" many times while switching HDMI's! Could it be also a video codec-related problem (probably playing non-stop the streaming mp4's but pausing the streaming mkv's and webm's as an example)?
Unfortunately this can happen sometimes with certain Windows PC's when attempting to manually flash a U22 & the cause isn't always immediately clear either. It's also not necessarily something you're doing wrong, it may just be your specific hardware.
First, it could be your cable. Preferably use the USB A -> USB C data cable that was supplied with the device. If that isn't working it's certainly worth quickly trying again with another cable just in case that was the problem initially. (It doesn't have to cost a fortune or be listed as high spec etc. (I frequently use a budget Amazon basics USB A -> C & that works fine flashing a U22, for example.))
Second, try
ALL of the available USB ports on your PC/laptop. (If it fails initially, disconnect everything, choose another USB port & begin again from step 1. Rinse & repeat until you've tried all available ports. (Sometimes one port may work & others won't.)). If you have one, you could even try a USB hub on the off chance the U22 is recognised through that.
Third, try another version of the USB burning tool. (The official guide states to use 2.2.0 or later, but I've been able to flash using a version as far back as 2.1.7.1/2.1.7.3.). There are also newer versions of the burning tool available here...
https://androidmtk.com/download-amlogic-usb-burning-tool
Otherwise, before you potentially start messing around disabling or even uninstalling things like security suites, firewalls, antivirus, or various other software on your main PC system....
....Do you have access to or can you borrow another PC just to quickly flash your device? Sometimes that's actually easier than the alternatives of arbitrarily removing software in the hope that whatever is "possibly" blocking the process suddenly allowing it through. (As some examples, I can't flash a U22 from an HP Omen gaming laptop or an MS Surface Pro running Win 10 Pro. However, I can successfully flash a U22 from a MINIX N42C-4 or J50C-4 Win 10 Pro devices. I also have an older Toshiba touchscreen laptop that's been kept around purely as an emergency backup (originally Win 8.1 but now Win 10), & I usually have to use that to flash my U22 because of the problems with my more recent hardware. All of these devices will, however, successfully flash a U9-H with any version of the USB burning tool I've tried.
If you know someone or even have an older desktop rig or laptop yourself that is at least Windows 7, you may have a significantly improved chance of success using one of them instead. (The flashing process only takes approx 5 minutes so you won't be imposing on a potential helper for long & obviously the software can be quickly uninstalled again once you're done if preferred.)
Give that lot a go & hopefully one of them will work sooner rather than later
.