It's possible this is caused by govim, as per @leitzler on Slack:

This setting might be useful for you unless you have it set already: https://github.com/govim/govim/blob/61d4fa23afba84a51e9a88a81c721907261bd8cb/cmd/govim/config/config.go#L312

And @heschi then mentioned:

The setting Pontus linked to is totally unsupported in gopls, but on by default in govim.

It would be helpful with a repro from my point of view, I can give it a look into the govim side.
I haven't used go.work yet so it is unfamiliar territory. This simple example did work for me where I jump from b.B in a/a.go to b/b.go:

-- a/a.go --
package a

import "b"

const A = 2

func fn() {
	b.B
}
-- a/go.mod --
module a

go 1.18
-- b/b.go --
package b

const B = 1
-- b/go.mod --
module b

go 1.18
-- go.work --
go 1.17

directory (
    ./a
    ./b
)

I do however the setting I linked in my config (the default value is 1/true): (EDIT: it works without it as well)

call govim#config#Set("ExperimentalAllowModfileModifications", 0)

Also had to enable workspace modules using:

call govim#config#Set("ExperimentalWorkspaceModule", 1)

Thanks for the report.

@mvdan have you tried call govim#config#Set("ExperimentalAllowModfileModifications", 0)? Does that avoid the errors for you?

I haven't had more time to dig into this or try again :) I also did not enable workspace support in gopls via call govim#config#Set("ExperimentalWorkspaceModule", 1). I honestly assumed that gopls would just work out of the box with it, given that cmd/go from Go master now supports workspaces by default. If I still need to enable the feature in gopls, maybe the instructions in the proposal should be updated.

I honestly assumed that gopls would just work out of the box

Agreed, which is why I'm suspicious of this govim setting. I can investigate and try to repro, but it will probably take a few days for me to get to this. If it's easy for you try unsetting this experimental feature in govim, that would help narrow it down.

Change https://golang.org/cl/347593 mentions this issue: cmd/go: print offending -mod value in workspace mode

Ever since I disabled ExperimentalAllowModfileModifications, I haven't run into this error again. So I think the blame should indeed be put on that option being on when it shouldn't be. I've filed govim/govim#1112 to fix that in govim.

I think we can close this issue for now, unless others run into similar errors with gopls again (with that experimental setting turned off!).