So good news and bad news:
Good news is that I made some progress in getting the store/deposit back to work. I was able to create a preliminary list of items that need to be updated due to changes from 1.12 to 1.13. There are some items that are not automatically assignable for the conversion of 82 items. In order to shorten that list I will run that 82 item list against the actual database to find the items that are actually present in the system and then write a manual block conversion list for the leftovers.
Bad news is that Websend seems to be broken in much moreplaces than previously thought. This is extremely frustrating since my Java knowledge and my knowledge about this plugin is just miserable. So all I do takes 10x longer than with the stuff I wrote myself. Right now I managed to find out how to add much more detailed debug info into Websend so that I can track down better where errors are generated. This has led me to this issue here.
I hope that someone at spigot can help me how to solve this.
If you know any Java, please look into the above issue. I can use any help I can get.