I am frequently experiencing an error in my SharePoint 2013 on premise environment when users try to upload a document to a document library. When a user uploads a document, the person is asked to fill out two fields (both are lookups). Afterward, the workflow should send the document to a 'To Be Approved' folder. However, this is not happening for 'some' documents so they get stuck. Not every document is failing to move. A developer created a custom Visual Studio workflow so the solution is deployed as a .WSP to the farm. I do not have access to the source code. The attached .PNG file shows the error that is logged in a custom list called 'Logs' in SharePoint. However, I obtained the ULS logs for that time and I have added that below. The build information is Windows Server 2012 R2 (Build 9600) with SharePoint 2013 v.15.0.4727.1000. Any help / guidance is much appreciated. Further, how can one see the source code in the .WSP file.
Note: Below, I have changed the user's staff id to 'user id' and I have re-named the .pdf file.
ttidLogWssCellStgConcise [Url: Incoming Documents/test123.pdf][User: i:0#.w|user id (NonCobaltOriginated)][StreamSizes: Request=0 bytes, Response=0 bytes][TotalRequestTime: 16msec][PartitionID: Default][Request: PutChanges **FFU Pri=0 Result=Success][cbOld=0][cbNew=111464][cLR:0][MD:0=AutoGenerated.OOF.No.NoOtherUsers.FutureOpen.Recently.DocumentIsActive.Content.Wired.None.Normal][cBCQ=0][cbCQ=0][cBRB=0][cbRB=0][cbREB=0][cBHBW=4][cbHBW=113262][cbHBEW=257280][cBHBDel=0][Mngd]