Thank you @Fabby,
mhwd-kernel --listinstalled
Currently running: 5.10.56-1-MANJARO (linux510)
The following kernels are installed in your system:
* linux510
* linux54
During boot, I went to options and chose the 5.4 version. Then startup is delayed by start-jobs for three USB-devices, that are not mounted actually as they are used for Backups only. Back to Kernel 5.10, there is no issue like that.