After Upgrading to 11.1 boot fails from time to time

At some point of booting the computer reboots.

After one or two such reboots, I get a complete booting.

Is there a log file to see after the successful booting the cause of the failing bootings?
The first file to look: /var/log/messages. Although, depending on the nature of the crash, there may not be a lot to go on.
In the same log file, check the one just before that. And if there's just that one maybe it's a new log file. Check an older log file. It's compressed and stored in /var/log/ too.

Or maybe nothing is being logged.
In /var/log/messages are messages from yesterday followed by messages of the successful boot of today, the messages of the failed boots of today are not in between.
# ll /var/crash/
total 1
-rw-r--r--  1 root  wheel  5 Sep 29  2016 minfree
# cat /var/crash/minfree
Then it probably crashes before the system has been initialized. So you're going to have to catch the error as it's happening.
The last device that I see recognized wheen boot fails is the Mouse. Here the messages from a succesfull booting around mouse:

Aug 13 06:35:19 fbsd kernel: uhub3: 5 ports with 5 removable, self powered
Aug 13 06:35:19 fbsd kernel: ugen1.2: <Ralink 802.11 n WLAN> at usbus1
Aug 13 06:35:19 fbsd kernel: ugen7.2: <vendor 0x0cf3 product 0x3000> at usbus7
Aug 13 06:35:19 fbsd kernel: ugen0.2: <Logitech USB Optical Mouse> at usbus0
Aug 13 06:35:19 fbsd kernel: ada0 at ahcich2 bus 0 scbus2 target 0 lun 0
Aug 13 06:35:19 fbsd kernel: ada0: <TOSHIBA MK2555GSXF FH405B> ATA8-ACS SATA 1.x device

Is the mouse or ada0 the problem?