I have the same problem was able to fix by rolling back the latest neovim update.

You mean I should rollback to nvim 0.9.5? When your car has a flat tire, you replace the tire not the car.

Several of the items in the compare module are tables with __call metamethods rather than simple functions. The recently_used comparator is one of those.

Commenting that one out in my comparators list fixed the problem for me.

Commenting that one out in my comparators list fixed the problem for me.

Yeah, of course. Just another "workaround". I don't want to downgrade neovim, I don't want to "hide" the problem by commenting it out. I want it to be fixed. That's why I opened this. In fact, I already know the problem is related with comparators:

When I open nvim with some comparators defined in nvim-cmp, they are giving me error in the title.

Instead of suggesting workarounds please upvote the issue so that it can get some attention from developers.

OK. I have reproduced.

It seems misc.merge() is broken.

misc.merge = function(tbl1, tbl2)
  local is_dict1 = type(tbl1) == 'table' and (not islist(tbl1) or vim.tbl_isempty(tbl1))
  local is_dict2 = type(tbl2) == 'table' and (not islist(tbl2) or vim.tbl_isempty(tbl2))
  if is_dict1 and is_dict2 then
    local new_tbl = {}
    for k, v in pairs(tbl2) do
      if tbl1[k] ~= misc.none then
        new_tbl[k] = misc.merge(tbl1[k], v)
      end
    end
    for k, v in pairs(tbl1) do
      if tbl2[k] == nil then
        if v ~= misc.none then
          new_tbl[k] = misc.merge(v, {})
        else
          new_tbl[k] = nil
        end
      end
    end
    return new_tbl
  end

  if tbl1 == misc.none then
    return nil
  elseif tbl1 == nil then
    return misc.merge(tbl2, {})
  else
    return tbl1
  end
end

misc.merge brokes metatable. __call is removed. So it will be error.

I have fixed it in #1956. Please test.

@sahinakkaya cmp.config.compare.sources does not exist. I think you can fix the error by removing nil-containing lists from your config.

local comparators = {
    cmp.config.compare.exact,
    cmp.config.compare.locality,
    cmp.config.compare.recently_used,
    cmp.config.compare.score,
    cmp.config.compare.sources,  -- == nil
    cmp.config.compare.offset
}
assert(vim.islist(comparators) == false)

misc.merge() detects dict-like tables with vim.islist() and merges them, so comparators field (list-like table) and its elements can not to be merged.
But if you put nil in the list-like table, vim.islist() returns false and then misc.merge() tries to merge comparators.

OK. It seems configuration bug.
cmp.config.compare.sources does not exist(it is exists once?).

But I think nil check is needed for it...