Got an answer from Apple: There has been an issue identified starting iOS 15.3, where the view through APIs may return an "unknown error", in cases when no error has actually occurred. In 15.3 that error is ASDErrorDomain:500. In 15.4 that error is now defined in the public SDK as SKANErrorDomain:SKANErrorUnknown (value 10). However, in those cases no functional error has actually occurred. The impression was recorded by the system. We recommend that you ignore all errors of type ASDErrorDomain:500 / SKANErrorDomain:SKANErrorUnknown (value 10), as they are not real errors. The impression is recorded on the device regardless of those errors.

iOS 15.4 RC, an issue is still there

iOS 15.4 beta 5 Mar 2 12:54:05 TestApp(StoreKit)[752] : SKAdNetwork: Error in remote proxy while starting view-through impression: Error Domain=SKANErrorDomain Code=10 "(null)" Mar 2 12:54:07 TestApp(StoreKit)[752] : SKAdNetwork: Error in remote proxy while ending view-through impression: Error Domain=SKANErrorDomain Code=10 "(null)" Mar 2 12:54:24 TestApp(StoreKit)[752] : SKAdNetwork: Error in remote proxy while ending view-through impression: Error Domain=SKANErrorDomain Code=4 "(null)"

iOS 15.4 beta 3 Feb 16 11:29:39 TestApp(StoreKit)[461] : SKAdNetwork: Error in remote proxy while starting view-through impression: Error Domain=SKANErrorDomain Code=10 "(null)" Feb 16 11:29:44 TestApp(StoreKit)[461] : SKAdNetwork: Error in remote proxy while ending view-through impression: Error Domain=SKANErrorDomain Code=10 "(null)" Feb 16 11:29:44 TestApp(StoreKit)[461] : SKAdNetwork: Error in remote proxy while ending view-through impression: Error Domain=SKANErrorDomain Code=4 "(null)"

in iOS 15.4 beta 2 it's became Error Domain=SKANErrorDomain Code=7 "(null)"