I fixed it. I opened the RPP file in a text editor, then search for "Deelay"
Code:
<VST "VST3: Deelay (Sixth Sample)" Deelay.vst3 0 "" 462967914{ABCDEF019182FAEB4D616E75576E6E65} ""
alSYG+5e7f4CAAAAAQAAAAAAAAACAAAAAAAAAAIAAAABAAAAAAAAAAIAAAAAAAAAvQcAAAEAAAD//xAArQcAAAEAAABWQzIhaAcAADw/eG1sIHZlcnNpb249IjEuMCIg
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IHZhbHVlPSIwLjAiLz48UEFSQU0gaWQ9IndldCIgdmFsdWU9IjEuMCIvPjwvUGFyYW1zPgAAAAAAAAAAAEpVQ0VQcml2YXRlRGF0YQABAUJ5cGFzcwABAQMAHQAAAAAA
AABKVUNFUHJpdmF0ZURhdGEAAAAAAAAAAABQcm9ncmFtIDEAEAAAAA==
>
Apparently this bit on the first line:
Code:
462967914{ABCDEF019182FAEB4D616E75576E6E65}
is some sort of ID of the plugin. All lines below it are (probably) the settings.
I added an instance of Deelay to the track (which does load), then copied the "ID" (which was indeed different) from its first line in the text editor and pasted it over de first line of the Deelay instance that didn't load.
The project opens without issue, and the plugin instance that did not load, now does load, now has the correct settings!
A bit of a hassle but it's fixed. There was no way I could have recalled the correct settings for Deelay from my memory.