Since there was few inner memory, I set SD-card as default for saving photos. However, something went wrong and photos of the last month of using it were not on the SD-card.
The restarting loop began after an SMS arrived.
Thus the most probable cause for restarting problem is that inner memory got almost full by photos saving, and that SMS message overfilled it. The operating system does not know what to do and keeps restarting the phone.
As soon as device connection sound plays on a computer, the phone gets restarted. So there is no way to connect it with loaded operating system via USB port.
In download mode it is recognized by Windows 10 as "mobile usb modem 1.0" and thus there is no access to its inner memory, including with STune 1.07 and TkFileExplorer 2.4 programs. Multiloader 5.67 finds it, but flashing using it will delete the photos and contacts.
Please let me know how can I recover the photos and contacts?
I see three possible directions for thought:
1. flash it with preserving user data option,
2. access inner memory while the phone is connected in download mode,
3. create a back-up copy of current ROM and later open it up on the computer.