I am on RPGMZ version 1.9.1 and I too am having this same issue. I just updated to the newest version of the plugin and did not have this issue before updating :(
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Hi, in the editor, can you click Game -> Update Corescript and then select 1.9.0 in "Version to Overwrite" and click ok, then save your project and let me know if it fixes the issue? As far as I am aware, there is no 1.9.1 core script, maybe you are confusing the editor version with the core script version?