V3.8.7 is now out for the 2860, no fixes for the dashboard invalid date but the authorisation fault has disappeared. Actually feels like a v3.8.6.1 with no new function and only a few issues resolved.

Improved: Improve stability of LTE module operation (for 'L' models).
Improved: Add TR-069 parameters for LTE ping keep alive.
Improved: Add auto reboot function for Vigor LTE WAN.
Corrected: Outbound SIP call disconnected after specific ITSP was used for 20 seconds.
Corrected: In PPTP VPN connection, remote dial-in user could not get DNS server IP
from its LAN subnet.
Corrected: DHCP Server stopped working, when "Block IP packet from LAN with
inconsistent source IP addresses" was enabled for IP Spoofing Defense option.

Other than that, seems to work ok for me.

I was just about to report two bugs to Draytek ... 1) wrong DNS server address being given to Dial-in users and 2) DHCP sometimes ignores Bind 2 MAC settings.

Since both DHCP and Dial-in DNS get a mention in the release notes, I suppose I'd better try this version first ;-)

Oh jeepers, I use both of those functions, so I had better look a bit harder, thanks for the heads up.

They are both bugs in 3.8.6 ...

It was dishing out my ISP's DNS server addresses (which I don't use anywhere) ... this appears to be fixed in 3.8.7. The release notes only mention PPTP (which I wouldn't dream of using in 2018!) ... the problem was definitely occurring with L2TP/IPSec as well.

The Bind 2 Mac issue is harder to test, since it happens at random. At 05:15 this morning, my 'Samknows Whitebox Tester' and (useless) Netatmo thermostat were both given random IP addresses (though from the correct scope). The Bind 2 MAC settings were ignored, but since Strict Bind is set, this immediately stopped the pair of them communicating with the outside world

Very odd, I haven’t experienced either of those issues, I was force using the Google NS and it’s still ok after the new build went on. As far as bind2mac goes then I have about 60 devices on the network, all DHCP but bound to a specific address in the table. I haven’t found any discrepancy yet.
I assume you have tried unsetting and setting the functions again?

Interestingly, I cant' seem to find it on the Download site, and Firmware checker still only offers 3.8.6