Branch users cannot load the intranet over the new GRE tunnel, but ping works

Last weekend the branch office moved from an MPLS circuit to a GRE tunnel over the internet between R2 (branch) and R1 (HQ). Since Monday, branch users say the intranet site on SRV1 (10.10.0.20, HTTPS) never loads: the browser just spins. Ping from PC1 (10.30.0.10) to SRV1 works. Uploads from the branch to a file share on SRV1 work. HQ users have no problems with the same site.