phpactor: Doesn't appear to do anything, with mysterious "ERROR" in :LspLog

I’m trying to move from a standard nvim setup to nvim IDE. I’ve used kickstart.nvim, and after installing phpactor when I edit a php file I get no autocompletion. :LspLog shows:

[START][2023-01-04 12:11:23] LSP logging initiated
[ERROR][2023-01-04 12:11:23] .../vim/lsp/rpc.lua:733    "rpc"   "phpactor"      "stderr"        "Starting language server, use -vvv for verbose output\n"

Which seems odd to me, as the line that says [ERROR] doesn’t appear to obviously actually BE an error…? But maybe someone here who knows more than me can point me in the right direction?

:LspInfo shows:

Language client log: /root/.local/state/nvim/lsp.log
Detected filetype:   php

1 client(s) attached to this buffer:

Client: phpactor (id: 1, bufnr: [1])
 filetypes:       php
 autostart:       true
 root directory:  /var/www/sites/myproject
 cmd:             phpactor language-server

Configured servers list: phpactor, sumneko_lua

If I edit the init.lua file, I DO get autocompletion (with a very similar :LspInfo output:

Language client log: /root/.local/state/nvim/lsp.log
Detected filetype:   lua

1 client(s) attached to this buffer:

Client: sumneko_lua (id: 1, bufnr: [1])
 filetypes:       lua
 autostart:       true
 root directory:  /var/www/sites/myproject
 cmd:             lua-language-server

Configured servers list: phpactor, sumneko_lua

…so the issue seems to lie with phpactor rather than nvim/lsp/mason/whatever (I’m a complete noob to this, no idea what I’m talking about…)

Thank you!