Alright so I was able to tease out more info by running this as a pipeline job.

So when I run

    require(xlsx)
    write.xlsx(labkey.data, file = "${fileout:labkeyData.xlsx}")

This is what I get

13 Sep 2016 16:43:10,566 INFO : Running R report job 'Test'
13 Sep 2016 16:57:03,225 ERROR: Error occurred running the report background job
javax.script.ScriptException: javax.script.ScriptException: javax.script.ScriptException: An error occurred when running the script 'script.R', exit code: 1).
Loading required package: xlsx
Loading required package: rJava
Loading required package: xlsxjars
Warning messages:
1: package 'xlsx' was built under R version 3.2.3
2: package 'rJava' was built under R version 3.2.3
3: package 'xlsxjars' was built under R version 3.2.3
Error in .jcheck(silent = FALSE) :
  Java Exception <no description because toString() failed>.jcall(row[[ir]], "Lorg/apache/poi/ss/usermodel/Cell;", "createCell", as.integer(colIndex[ic] - 1))<S4 object of class "jobjRef">
Calls: write.xlsx ... .write_block -> createCell -> .jcall -> .jcheck -> .Call
Execution halted
Caused by: javax.script.ScriptException: javax.script.ScriptException: An error occurred when running the script 'script.R', exit code: 1).