The RadScheduler does not care about the length of the recurrence rules, so you can set the field size to an even bigger number.
There might be a possibility if when trying to save the recurrence rule, the SQL cut the text and that would lead to an invalid recurrence rule.
Please try creating a new appointment to the data base and start adding exceptions until the issue is observed again. It should work properly until the limit of the column size is met.
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