If you have tried running a year's worth of data prior to this (a month ago?) with no problems, it could simply be that this one additional month's worth of data has 'broken' this limit! There are more records now to report on, not less.

Can you 'print preview' the report without a problem (i.e. is it just the export process that will not work as before)?

Can you otherwise limit the report in some other way, for example - print one report for 1/2 your customers, and print another report for the other 1/2?