Hi Sam,
I have had the same sort of problem.
I updated to Pi-hole Version v3.0.1 Web Interface Version v3.0.1 FTL Version v2.7.3
FTL always says off-line, and web updates are super slow.
I tried un-installing and re-installing.
I also tried the suggestions above, pihole -r. It all runs ok, but still no FTL
Any ideas?
John