Generally, plugins use address space outside Photoshop. That's one of the reasons you should never set memory allocation too high in Photoshop. There won't be anything left for other apps and plugins.
This sounds either like a bug in the plugin, or your disk is full and Photoshop can't allocate enough scratch disk space. Photoshop will not shut down or fail in low memory conditions. It will just write to disk instead (scratch disk). That's a bit slower, but everything will still work.
Adobe now only supports N-1 versions (current and previous). So when 2021 came out, that removed 2019 from the list. 2020 can still be installed from the CC desktop app.