Persistence

My FTP or PPTP session initiation sometimes fails (persistence)

Those protocols are using a second random TCP port. The session initiation is rejected by the remote host if the two sessions are balanced on two links. To ensure this second port will be captured in the layer 3-7 session initiation and use the same link, configuration should include the protofix statement and an ACL NAT IN +IP statement. The ACL NAT IN +IP statement will ensure the second TCP session passes through the algorithm decision engine and not in passthrough.
For example with a firewall 24.201.1.3 IP address, FTP transferts in round-robin and all other protocols in OPFA:
protofix ftp
acl nat in +tcp 1 +24.201.1.3/32:0-0 +0.0.0.0/0:21-21 +nat poolip:1,23 rr
acl nat in +ip 1 +24.201.1.3/32 +0.0.0.0/0 +nat poolip:1,23 opfa

Only one particular website sometimes reject my session.

Some security devices on the market perform a DNS reverse lookup on your firewall browsing IP and do not support multi-homing. If you use the same public IP address for web browsing and IDNS interception or incoming services, the remote site security device could do a reverse lookup and find the IP address of your second link. The session is closed by the remote site.

For this reason, we recommend to reserve the web browsing IP addresses for all your links and use other IP pools for IDNS interception and incoming services. If you are not able to reserve web browsing IP addresses on all your links, you can create an acl nat in statement specifically with the web site destination IP with OPFA algorithm.

Another very important verification is to have persistency for the port 80 and 443 in your Link LB configuration:
acl per in +tcp 1 +0.0.0.0/0:0-0 +0.0.0.0/0:443-443 +persist 600
acl per in +tcp 2 +0.0.0.0/0:0-0 +0.0.0.0/0:80-80 +persist 600