Many Payment Aggregators, Payment Gateway firms, Credit Card issuers, Banks n other players in the payments-settlement space DO NOT HAVE their systems/servers running on a 24x7 basis. Even though they claim so, in reality, many do have a EOD-SOD down time in their so-called fully online service models.
So, to avoid such trxn-drops n failed Trxns, I always do my Trxns during 9-to-6 to the extent possible.
For ex. Today at 01.00 am, I tried to send money from my SBI a/c to my HDFC a/c by using UPI-id. It failed with the remark - invalid id. The same trxn, repeated exactly at 10.00 am today went off without any issue. The problem could be at either end or anywhere in between - as more than 6 players are involved in each trxn. Anyone's server is down means the trxn is down. SBI n HDFC will have (I know they have) full hot-standby, mirrored sites n can handle non-stop 24 hour business. But, the very many intermediaries are NOT equipped to do so.