Today (actually yesterday), I also had a DNSSEC problem (SERVFAIL in the pihole log) for www.paypal.com and some related domains. Using dnsec on unbound, NOT on pihole-FTL.
I restarted unbound and the problem was gone, no idea what the cause is / was.
I regulary run into (read) topics (not just on this forum) that mention unbound doesn't work for a single (just one) domain, strange...