Unfortunately, all the dates for a recurring event are stored in one database text field, and there is a limit as to the size of that text field. It actually used to be even smaller, but when we couldn’t add enough dates for one of our school’s events, I increased that field type to VARCHAR(500).
I’m not sure how much larger you could make that field without it slowing things down, but you could try manually editing the database fields definitions, and perhaps switch it to type LONGTEXT, which is what is used for the sheet description (like a normal post content field). It’s the ‘dates’ field in the tasks table, which is the second database table. Definition is in the main plugin file on line 382.
You can increment the database version number to force it to update, or deactivate/reactivate, and that should update the tables with any changes you make. Could also try updating that field type with any database tools you have for your server, such as PHPMyAdmin.