Issue on downloading zip archive on shinyapps.io

Hello,
It seems not possible to archive and download files in a single zip file when working on shinyapp.io server.
The implemented procedure works perfectly running locally but I got this problem looking into the logile provided by the admin interface:

...
*2020-01-28T15:14:19.144899+00:00 shinyapps[1715264]: if (inherits(e, "shiny.silent.error")) *
2020-01-28T15:14:19.151502+00:00 shinyapps[1715264]: 86: stop
*2020-01-28T15:14:19.144895+00:00 shinyapps[1715264]: Called from: (function (e) *
2020-01-28T15:14:19.151505+00:00 shinyapps[1715264]: 84: observeEventHandler [/srv/connect/apps/cytoChain24/server.R#1975]
*2020-01-28T15:14:19.144900+00:00 shinyapps[1715264]: if (is.function(handle)) *
2020-01-28T15:14:19.151504+00:00 shinyapps[1715264]: 85: zip
2020-01-28T15:14:19.144897+00:00 shinyapps[1715264]: {
2020-01-28T15:14:19.151506+00:00 shinyapps[1715264]: 13: runApp
2020-01-28T15:14:19.144899+00:00 shinyapps[1715264]: return()
2020-01-28T15:14:19.151506+00:00 shinyapps[1715264]: 12: fn
2020-01-28T15:14:19.144899+00:00 shinyapps[1715264]: handle <- getOption("shiny.error")
2020-01-28T15:14:19.151507+00:00 shinyapps[1715264]: 7: connect$retry
2020-01-28T15:14:19.144900+00:00 shinyapps[1715264]: handle()
2020-01-28T15:14:19.151507+00:00 shinyapps[1715264]: 6: eval
*2020-01-28T15:14:19.144901+00:00 shinyapps[1715264]: })(list(message = "'files' must a character vector specifying one or more filepaths", *
2020-01-28T15:14:19.151507+00:00 shinyapps[1715264]: 5: eval
2020-01-28T15:14:19.144901+00:00 shinyapps[1715264]: call = zip(zipfile = "./tmpdata/plotMeta.zip", files = Zip_Files)))
2020-01-28T15:14:19.164203+00:00 shinyapps[1715264]: Session stopped
2020-01-28T15:14:19.145505+00:00 shinyapps[1715264]: Warning: Error in zip: 'files' must a character vector specifying one or more filepaths
...

It looks like no files was generated in the server's temporary directory.

On the Server side the code is:

path_to_file <- "./tmpdata"
Zip_Files <- list.files(path = path_to_file, pattern = ".png$", full.names = TRUE)
zip(zipfile = "./tmpdata/plotMeta.zip", files = Zip_Files)
        
output$downloadMeta <- downloadHandler(
          filename = function() {
            paste("output", "zip", sep=".")
          },
          
          content <- function(file) {
            file.copy("./tmpdata/plotMeta.zip", file)
          },
          contentType = "application/zip")

Could you provide any hint?

Try printing the value of Zip_Files and then checking your logs.
If it is empty, then you will need to debug the process by which .png files are written there.
See the documentation on logging, in particular, and troubleshooting, in general, for more tips.

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