I´m not sure what to say.. But this is rather annoying..
I just finished spending 40 minutes on installing a fresh openhab 2.5.7-1 on a SD card.. And guess what.. I got the very same issue again. Frontail logviewer is not running..
This time I noticed, at the end of the openhab installation, it installs the frontail logviewer, it also start the service, and a few second after, it stops the service and never starts it again..
That explain howcome it doesnt work.
using openhabian-config to “install” the logviewer will make it work, but not without issues. After restarting openhab, it still wont be running.
I´m not sure whom to tag on this one, but there are some issues on the image, thats for sure..
At least I had no better things to do the last 4-5 hours 
Btw.. This is suppose to be “stable” version..Hmm!