Last Update CM13 - Nexus 5X Yesterday I updated from this snapshot version 2016-04-18 17:30:08 to the last one 2016-08-20 00:21:07 (bullhead)

Everything looks in its place, but yesterday the device was keeping restarting, today it's stable.
I tried to reboot it by myself and when boot it says:

There’s an internal problem with your device. Contact your manufacturer for details.

Any solution?
I have found a soulution here but it's for Nexus 6P and Android 5.1 but I think it will work in your case as well.

Android System
There's an internal problem with your device. Contact your manufacturer for details.

The problem occurs because of a check that Google implemented in Android 5.1 (and it seems stayed in after) which compares /system/build.prop with the values found in /vendor/build.prop.
If they differ you get that error message. All one has to do to get rid of the error is to change the 3 values in /vendor/build.prop according to the values in /system/build.prop.
On the link you can find a flashable zip. The magic is in the update-binary file:
/sbin/busybox egrep &quot;^ro\.build\.(date(\.utc)?|fingerprint)=&quot; /system/build.prop \
    | /sbin/busybox sed -e&quot;s/^ro\./ro.vendor./&quot; \
    &gt; /vendor/build.prop

So you need:

install TWRP on your device
send vendor-build-prop-fix-signed.zip to your device
reboot into recovery mode
flash the zip but don't wipe anything
reboot your phone

If it's not work (but it should be work), you can try this method.
It says you should download and flash/install the vendor.img which matches your ROM's base.