NFS mount attempt with vers=n returns "invalid argument" or "incorrect mount option"

This is addressed in SLES 15 SP6 and SP7 within the nfs-client package v2.6.4.  This was initially fixed in upstream linux code in v2.5.3, but by the time SLES 15 SP6 was put together, the code and version number were further updated.

For SLES 15 SP5 or lower (including SLES 12 SP5), the recommended solution is not to set an NFS version in /etc/nfsmount.conf.

The code responsible for merging options together is found in mount.nfs, part of the nfs-client package, which is built from the nfs-utils source code package.  Use of /etc/nfsmount.conf and the code to merge its options with others is a relatively new development.

If different NFS versions are specificed in /etc/nfsmount.conf and elsewhere (such as /etc/fstab, on the command line, or possibly autofs maps), various logic comes into play to determine which version will be used.  This is further complicated by the fact that "vers=" is considered an alias of "nfsvers=" but could be handled differently by some code.  There have been combinations that work and some that fail, with various reasons and exceptions within the logic.

The package "nfs-client" is just one of several packages built from the source package "nfs-utils".  For example, "nfs-kernel-server" is also built from "nfs-utils".  Sometimes the name "nfs-utils" is used instead of the others, which can be accurate but still confusing, because "nfs-utils" is not a compiled package name that exists within current shipping SLES distributions.