You have duplicate alternate index keys. Each existing key is a record with the primary key of each record which matches that key. You now, today, have more duplicates for a particular value (at least one) than previously, and that happens to make your alternate index record too small. Make it bigger. If it is already maximum, then you have thinking to do which should have been done at the design stage. How did this get through testing?
It would help if you showed the BASE CLUSTER define statement.
The QI309 message is also unfamiliar, are you using a 3rd party product of basic IDCAMS to do the define?
Not sure how you can have RECORDSIZE(900 670000), i.e. a 670k recordsize!? I think the record size is limited to 32760 bytes. Also the CISIZE is only 4096 bytes and you haven't specified SPANNED so even a 32k record size wouldn't fit.
Suggest you read the IBM manuals 'VSAM Demystified' and 'Using Data Sets' urgently.