Error rpc_rejected

The server rejected the request at the RPC layer

The program version the app asked for isn't what the server offers. Usually a server that speaks only NFSv4 reached through the NFSv3 path, or Custom Port pointing at a different RPC service.

What this means

The server answered, but said the RPC program or version doesn’t match. NFS Files speaks NFSv3 (with the separate MOUNT v3 protocol) and NFSv4.0. This error means the server doesn’t offer the one the app tried.

Most likely: a version mismatch

• The server is NFSv4-only (TrueNAS with NFSv3 disabled, many hardened Linux setups, some Docker images). With NFS Version: Auto the app tries v4 first, then v3 — if the v4 attempt failed for some other reason, the error you see is from the v3 attempt and can be misleading. Set NFS Version → NFSv4 in the server’s Advanced settings so you see the real v4 error.
• The server is NFSv2-only. Very old devices and some routers with USB storage. NFS Files doesn’t support NFSv2; there’s no fix short of a newer server.
• NFSv4.1+ only. A server configured to require 4.1 or 4.2 will reject 4.0. On Linux, /etc/nfs.conf [nfsd] vers4.0=y re-enables it; on TrueNAS, check the NFS service’s protocol options.

Custom Port pointing at the wrong RPC service

If Custom Port is on, the port must be the NFS server (and for v3, also the MOUNT server). Pointing it at the portmapper (111) or another RPC program gives this error.

See what the server offers

Inspect Server on the server card shows the RPC table — every program and version registered on the server. Look for nfs versions 3 and 4 and mountd version 3.