IPN / Checking transaction / Access Denied

[Mavlutovr]

When my script try to check a transaction, it get the error:

Spoiler

Access Denied.
You don't have permission to access "/cgi-bin/webscr" on this server.
Reference #1626787044d6a297d5d938db06f7c42
Timestamp: 1626787044

I found a solution: add User-Agent.

But then I add to headers:

"User-Agent: IOctopus/1.0"
I get the error:

Spoiler

Bad Request
Your browser sent a request that this server could not understand.

Could you tell me, how can I fix it?

Url: https://ipnpb.sandbox.paypal.com/cgi-bin/webscr

Post Data: cmd=_notify-validate&insurance_amount=0.00&discount=0.00&transaction_subject=&txn_type=web_accept&payment_date=06%3A17%3A15+Jul+20%2C+2021+PDT&last_name=%1A%1A%1A%1A%1A%1A%1A%1A&residence_country=RU&pending_reason=unilateral&shipping_method=Default&item_name=IOctopus+%2F+Galaxy&payment_gross=&mc_currency=RUB&business=sandbox%40ioctopus.online&payment_type=instant&protection_eligibility=Ineligible&verify_sign=AlmexrYiFQQhDdaXJ75wYq5PialHAj-YqJGcpcuHrC-oFTIgTfP5oInM&payer_status=unverified&test_ipn=1&payer_email=paypaltest%40ioctopus.online&txn_id=2X047555N89183700&quantity=1&receiver_email=sandbox%40ioctopus.online&first_name=%1A%1A%1A%1A%1A&payer_id=J92NS5GKLTQLE&item_number=317047%3Ae8176b34d6ddb842fd49236b8ca3d0ed&payment_status=Pending&mc_gross=167.90&custom=&charset=windows-1252&notify_version=3.9&shipping_discount=0.00&ipn_track_id=4827936c3e7bf