The endpoint https://testnet.algoexplorerapi.io is deprecated:
AlgoExplorer API v2
You need to use the new endpoints.
Note however that /v2/accounts us no more supported by algoexplorer API.
You either need to use the indexer equivalent endpoint or use another API provider.
For example:
https://testnet-api.algonode.cloud/v2/accounts/ZRVP5276H7PWMI5VIQVLFGICYEOAUVD467FJ2Z72UUGDETF6K7LXBRHQ4E works
See https://algonode.io