In message https://stat.ethz.ch/pipermail/r-help/2017-August/448539.html Christopher Walker reported the following bug: readRDS can't read from an ascii serialization on a textConnection. The code below worked for him in 3.2.x and for me in 3.3.2 but not in 3.4.1.
> zz = textConnection('tempConnection', 'wb')
> saveRDS(c("a", "b", "c"), zz, ascii = TRUE)
> serialized_obj = paste(textConnectionValue(zz), collapse='\n')
Error in gzcon(file) : 'read' not enabled for this connection
This is an R 3.4.1 - only bug --- introduced when we made readRDS(url(.))
It is fixed in R-devel now and will be ported to "R 3.4.1 patched" soon.