Yes, this is an old problem, still impossible to upload the file that is larger than 2mb to the own/NextCloud behind the reverse proxy, it ends up like this:
2019-09-26 14:02:03.309 [INFO] WebDAV (WebDAV) : Check Change Thread Start
2019-09-26 14:02:37.305 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:02:37.320 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:02:39.102 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:02:39.102 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:02:41.836 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:02:41.836 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:02:46.633 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:02:46.633 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:02:55.492 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:02:55.492 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:03:12.288 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:03:12.288 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:03:45.084 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:03:45.099 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:04:49.844 [ERROR] WebDAV API : WebDAV : Upload AddContentFromCache Fail : Unable to write data to the transport connection: An existing connection was forcibly closed by the remote host.
2019-09-26 14:04:49.844 [ERROR] UPLOADFILE : UploadData Fail : Position 0 Length 8388608 ChunkCount 8 Finished False Exception : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"}
2019-09-26 14:04:49.844 [ERROR] UPLOADFILE : Finally UploadData Fail : Position 0 Length 8388608 Finished False
2019-09-26 14:04:49.859 [ERROR] WEBDAVFILE FILE : /Test file 22mb.pdf : WriteFile : Upload and wait : Failed : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"Upload AddContentFromCache Fail.","Filename":"/Test file 22mb.pdf","SocketError":"ConnectionReset","InnerException":"An existing connection was forcibly closed by the remote host"} : OpenFileInfo : IsDirectory:False, IsReadSequential:False, IsWriteSequential:True, IsReadMode:False, IsWriteMode:True, Attributes:134217760
2019-09-26 14:04:49.859 [ERROR] WebDAV (WebDAV) : Write Fail : /Test file 22mb.pdf : Error : 1237 : {"WinErrorCode":1237,"NotifyErrorCode":204,"RaiException":"{\"WinErrorCode\":1237,\"NotifyErrorCode\":204,\"RaiException\":\"Upload AddContentFromCache Fail.\",\"Filename\":\"/Test file 22mb.pdf\",\"SocketError\":\"ConnectionReset\",\"InnerException\":\"An existing connection was forcibly closed by the remote host\"}","Filename":"/Test file 22mb.pdf"}
2019-09-26 14:04:50.547 [ERROR] WEBDAVFILE FILE : /Test file 22mb.pdf : Final : File is modified but data not filled : Canceled : isClose : True : OpenFileInfo : IsDirectory:False, IsReadSequential:False, IsWriteSequential:False, IsReadMode:False, IsWriteMode:True, Attributes:134217760