NFS4, RPC struct is bad?

Hi,

I now and then have "RPC struct is bad" when extracting a big file over NFS.

Server: 14.4 VNET jail on a 16.0 aarch64 Raspberry Pi 4.
rc.conf:
nfs_server_enable=YES
mountd_enable=YES
nfsv4_server_only=YES
nfs_server_flags="-t -h pkg-nfs.thuis.klop.ws"

Client: 15.0 bhyve on 15.0 amd64 host.
I have /data/distfiles mounted over NFS.

The network path is
NFS-Server -> epair11b -> bridge0 -> genet0 -> physical switch -> em0 -> bridge0 -> epair2 -> bridge vm-public -> tap1 -> vtnet0 -> NFS-Client

The server jail and the host run an ipfw firewall.

[00:00:49] =======================<phase: checksum       >============================
[00:00:49] ===== env: DEVELOPER_MODE=yes STRICT_DEPENDS=yes USER=root UID=0 GID=0
[00:00:49] ===>  License EPL accepted by the user
[00:00:49] ===>   eclipse-4.39 depends on file: /usr/local/sbin/pkg - found
[00:00:49] ===> Fetching all distfiles required by eclipse-4.39 for building
[00:00:49] => SHA256 Checksum OK for eclipse/chirontt-eclipse.platform.releng.aggregator-4.39-R4_39_maintenance_GH0.tar.gz.
[00:00:49] => SHA256 Checksum OK for eclipse/eclipse-jdt-eclipse.jdt-R4_39_GH0.tar.gz.
[00:00:49] => SHA256 Checksum OK for eclipse/eclipse-jdt-eclipse.jdt.core-R4_39_GH0.tar.gz.
[00:00:49] => SHA256 Checksum OK for eclipse/eclipse-jdt-eclipse.jdt.core.binaries-R4_39_GH0.tar.gz.
[00:00:49] => SHA256 Checksum OK for eclipse/eclipse-jdt-eclipse.jdt.debug-R4_39_GH0.tar.gz.
[00:00:49] => SHA256 Checksum OK for eclipse/eclipse-jdt-eclipse.jdt.ui-R4_39_GH0.tar.gz.
[00:00:49] => SHA256 Checksum OK for eclipse/chirontt-eclipse.pde-R4_39_maintenance_GH0.tar.gz.
[00:00:50] => SHA256 Checksum OK for eclipse/chirontt-eclipse.platform-R4_39_maintenance_GH0.tar.gz.
[00:00:50] => SHA256 Checksum OK for eclipse/chirontt-eclipse.platform.swt-R4_39_maintenance_GH0.tar.gz.
[00:00:50] => SHA256 Checksum OK for eclipse/chirontt-eclipse.platform.ui-R4_39_maintenance_GH0.tar.gz.
[00:00:50] => SHA256 Checksum OK for eclipse/chirontt-equinox-R4_39_maintenance_GH0.tar.gz.
[00:00:50] => SHA256 Checksum OK for eclipse/chirontt-equinox.binaries-R4_39_maintenance_GH0.tar.gz.
[00:00:50] => SHA256 Checksum OK for eclipse/chirontt-p2-R4_39_maintenance_GH0.tar.gz.
[00:00:52] => SHA256 Checksum OK for eclipse/NorbertXYZ-eclipse-maven-4.39_GH0.tar.gz.
[00:00:53] install: /data/distfiles/eclipse/INS@j3KyQ0: RPC struct is bad
[00:00:53] =>> Cleaning up wrkdir
[00:00:53] ===>  Cleaning for eclipse-4.39


# nfsstat -m
pkg-nfs.thuis.klop.ws:/data/distfiles on /data/distfiles
nfsv4,minorversion=2,tcp,resvport,nconnect=1,soft,intr,cto,sec=sys,acdirmin=3,acdirmax=60,acregmin=5,acregmax=60,nametimeo=60,negnametimeo=60,rsize=65536,wsize=65536,readdirsize=65536,readahead=1,wcommitsize=16777216,timeout=120,retrans=2147483647

I have this enabled:
vfs.nfs.enable_uidtostring=1
vfs.nfsd.enable_stringtouid=1


Of course the trouble is that it does not happen every time.
Apart from this error the network is stable for a home-situation. Iperf runs fine for a long time.

Any thoughts?

Regards,
Ronald.