fileUploadField() returns 403 Error when upload too many documents

Hi every one, 

I'm receiving above error when using fileUploadField() to upload around more than 150 documents. I checked and each document I uploaded has size around 322.74 KB only.

Within my fileUploadField() below is the code: 

a!fileUploadField(
                          maxSelections: 250,
                          label: "Upload Document",
                          labelPosition: "COLLAPSED",
                          value: ri!documents,
                          saveInto: {
                            ri!documents,
                            /*Save document details */
                              a!submitUploadedFiles(documents: ri!documents),
                            a!save(
                              ri!uploadedDocuments,
                              reject(
                                fn!isnull(_),
                                a!forEach(
                                  items: save!value,
                                  expression: if(
                                    contains(
                                      tointeger(
                                        index(
                                          ri!uploadedDocuments,
                                          "appianDocumentId",
                                          null
                                        )
                                      ),
                                      tointeger(fv!item)
                                    ),
                                    displayvalue(
                                      fv!item,
                                      tointeger(
                                        index(
                                          ri!uploadedDocuments,
                                          "appianDocumentId",
                                          null
                                        )
                                      ),
                                      ri!uploadedDocuments,
                                      null
                                    ),
                                      a!map(
                                        appianDocumentId: tointeger(fv!item),
                                         /* Query to DB to search for the document type Id using document name, return null if there is no matching query output */ 
                                        documentTypeId: index(rule!PRO_RULE_GetDetailsFromTaxDocumentName(documentName: rule!GBL_RULE_getDocumentContentName(documentId: tointeger(fv!item))), "documentTypeId"),
                                        entityTypeId: cons!NWL_INT_ENTITY_TYPE_ID_ASSET,
                                        /* Query to DB to search for the entity ID using document name, return null if there is no matching query output */ 
                                        entityId: index(rule!PRO_RULE_GetDetailsFromTaxDocumentName(documentName: rule!GBL_RULE_getDocumentContentName(documentId: tointeger(fv!item))), "entityId", null))
                                  )
                                )
                              )
                            ),
                            a!save(
                              local!docCategories,
                              reject(
                                fn!isnull(_),
                                a!forEach(
                                  items: save!value,
                                  expression: if(
                                    contains(
                                      tointeger(
                                        index(
                                          local!docCategories,
                                          "appianDocumentId",
                                          null
                                        )
                                      ),
                                      tointeger(fv!item)
                                    ),
                                    displayvalue(
                                      tointeger(fv!item),
                                      tointeger(
                                        index(
                                          local!docCategories,
                                          "appianDocumentId",
                                          null
                                        )
                                      ),
                                      local!docCategories,
                                      null
                                    ),
                                    a!map(
                                      appianDocumentId: tointeger(fv!item),
                                      categoryId: null
                                    )
                                  )
                                )
                              )
                            ),
                            a!save(ri!docCategories, local!docCategories),
                            /*Save filename */
                            if(
                              rule!AAA_CMN_IsEmpty(ri!documents),
                              {},
                              a!forEach(
                                items: ri!documents,
                                expression: a!save(
                                  ri!uploadedDocuments[fv!index].name,
                                  rule!GBL_RULE_getDocumentContentName(documentId: fv!item),

                                )
                              )
                            ),
                          },
                          target: cons!FOLDER_CASE_DOCUMENTS
                        )

Is there any limitation of this function except for file size limit is 1GB that I should be awared of? Or could you suggest me how can I debug this issue?