/usr/lib/bind/ldap.so: undefined symbol: cfg_parse_buffer2

I'm running Ubuntu 20.04 ARM64 on a Raspberry Pi 4.

I'm trying to start bind-dyndb-ldap with systemctl after configuring /etc/bind/named.conf as follows:

# enable LDAP backend database
dyndb "878-LDAP" "/usr/lib/bind/ldap.so" {
        server_id "s-hrt1-cw";
        uri "ldap://localhost";
        base "cn=dns,cn=etc,dc=home,dc=arpa";
        auth_method "simple";
        bind_dn "uid=dnsadmin,cn=users,cn=home,dc=arpa";
        password "xxx";
};

It fails and journalctl -xe shows the following message:

Feb 23 18:06:54 s-hrt0-cw named[453188]: failed to dynamically load instance '878-LDAP' driver '/usr/lib/bind/ldap.so': /usr/lib/bind/ldap.so: undefined symbol: cfg_parse_buffer2 (failure)
Feb 23 18:06:54 s-hrt0-cw named[453188]: dynamic database '878-LDAP' configuration failed: failure
Feb 23 18:06:54 s-hrt0-cw named[453188]: loading configuration: failure
Feb 23 18:06:54 s-hrt0-cw named[453188]: exiting (due to fatal error)

I read somewhere that this is a known problem that has been fixed in Groovy. Is there a backport from Groovy available? I have tried to compile the Groovy package sources on Focal, but as yet without success. I have bind9 and bind9-libs v9.16.12 installed. Looking at the dependencies, it would appear that libc6 is the problem (requires >= 2.32, installed 2.31). I haven't been able to find a description of what differentiates libc6 2.31 from 2.32...

I don't particularly want to upgrade away from the LTS 20.04, so I'd be grateful for any hints on how to solve this problem in 20.04.

Regards,
Steve