We have the same experience since first versions of ROS v6 on x86 and different CCRs. Accessing active tab on PPP menu in Winbox sometimes crashes ppp (pptp/l2tp/pppoe/etc ppp) connections with radius auth. I've did numerous supouts, email support but didn't get a reply stating that it is a long time know bug. Support suggested to tune our radius, make some more supouts and so on.
As a workaround I've added missing columns to PPP->Interfaces in Winbox (IP & uptime) and crashing is gone.
To fix connections after you get red simple queues and also red dynamic ip's you can use two scripts. This way you don't have to reboot the box after ppp crash. Also take in account that if you don't use the "Active Connections" tab there are almost no issues with "red simple queues" (maybe once in 6 month).
add interval=1m name=red_simple_remove on-event="/queue simple remove [find invalid=yes disabled=no]" policy=ftp,reboot,read,write,policy,test,password,sniff,sensitive start-time=startup
add interval=1m name=remove_dynamic_invalid_ips on-event="/ip address remove [find dynamic=yes invalid=yes]" policy=ftp,reboot,read,write,policy,test,password,sniff,sensitive start-time=startup