Today I built a new image for my RUT955, after git pull. And the RUT955 was in a continuous boot loop, after sysupgrade.
Fortunately, I could enter Teltonikas boot loader, (reset pressed during pwr up, and then http://192.168.1.1 in browser).
Flashing an older image, from 02-August, brought back my RUT to normal state.
I tried the flash of most recent image twice, once a sysupgrade -n -v -f files_to_keep.tar.gz
and once a vanilla sysupgrade -n -v. Both times router was in continuous boot afterwards.
Working-/Nonworking image differe only in
CONFIG_TARGET_ath79_generic_DEVICE_glinet_gl-x300b is not set
CONFIG_PACKAGE_libsasl2-sasldb is not set
CONFIG_PACKAGE_libreswan is not set
CONFIG_PACKAGE_ethtool-full is not set
CONFIG_PACKAGE_fuse-overlayfs is not set
So, it looks like that during previous week something changed regarding sysupgrade.
This is my working image: OpenWrt SNAPSHOT, r17247-022d1fe80f