incorrect number of dimensions output could not be returned

Question

Here is my R code. This code is deployed on machine learning server (Rserver) as web service.

mydata <- data.frame(stringsAsFactors = FALSE)
req <- POST("<<someurl>>",httr::add_headers("Content-Type" = "application/json;charset=UTF-8"),body = bdy, encode = "json", verbose())

json <- content(req, 'text')

SpanningTree <- fromJSON(json)

df1 <- data.frame(stringsAsFactors = FALSE)


df1 <- mydata[grep("Backup|Blocking", SpanningTree$Value),] 

I am calling this web service (batch wise) from remote machine (c# client) but getting error on above line saying " incorrect number of dimensions output could not be returned." (I don't get this error on demand ).

Even if i go to machine learning server(RServer) and call above model from r studio, error is same.