I removed all traces of PlayMaker from the project, and then started to re-import it all.
It threw up this error after the PlayMaker installation:
Assets/PlayMaker/Editor/PlayMakerProjectTools.cs(220,115): error CS0618: `UnityEngine.ParticleEmitter' is obsolete: `This component is part of the legacy particle system, which is deprecated and will be removed in a future release. Use the ParticleSystem component instead.'
Which I got around temporarily by commenting out that particular line of code. Obviously not a permanent solution, but it let me continue for the time being.
Then, I downloaded and imported Array Maker from the Ecosystem, at which point it gave me those same original errors again:
Assets/PlayMaker Utils/Editor/PlayMakerEditorUtils.cs(102,94): error CS0618: `HutongGames.PlayMakerEditor.FsmEditorUtility.GetActionScriptAsset(HutongGames.PlayMaker.FsmStateAction)' is obsolete: `Use ActionScripts.GetAsset instead.'
Assets/PlayMaker Utils/Editor/PlayMakerEditorUtils.cs(111,90): error CS0618: `HutongGames.PlayMakerEditor.FsmEditorUtility.GetActionLabel(HutongGames.PlayMaker.FsmStateAction)' is obsolete: `Use Labels.GetActionLabel instead.'
So it's clearly linked in some way to the PlayMaker Utils stuff that is imported when I import Array Maker.