[R-SIG-Mac] Problem updating packages

[zListserv]

In MacOS 10.12.3 (Sierra), I can't update packages (though it worked on the same computer in MacOS 10.11):

update.packages( 
     lib  = lib <- .libPaths()[1],
     pkgs = as.data.frame(installed.packages(lib), stringsAsFactors=FALSE)$Package,
     type = 'source')

Warning: unable to access index for repository https://cloud.r-project.org/src/contrib:
  cannot open the connection
Warning message:
In read.dcf(file = tmpf) :
  cannot open compressed file '/tmp/RtmpS3f5mS/file959038dcc8eb', probable reason 'No such file or directory'

---

My internet connection works fine; updating hasn't worked since I upgraded MacOS to 10.12 from 10.11.
Confusingly, on a new MacBook Pro with 10.12.3 & R 3.3.3, updating works without error.  It feels as though something changed in the upgrade and now I can't get to where I got before.

I'd appreciate help in resolving this.
---

Here's my configuration:

R version 3.3.3 (2017-03-06)
Platform: x86_64-apple-darwin13.4.0 (64-bit)
Running under: macOS Sierra 10.12.3

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets 
[6] methods   base     

other attached packages:
[1] HANDLS_0.30    zUtil_0.94     sysMods_0.01  
[4] foreign_0.8-67 car_2.1-3     

loaded via a namespace (and not attached):
 [1] Rcpp_0.12.8        maptools_0.8-40   
 [3] splines_3.3.3      MASS_7.3-45       
 [5] xtable_1.8-2       lattice_0.20-34   
 [7] minqa_1.2.4        plyr_1.8.4        
 [9] tools_3.3.3        nnet_7.3-12       
[11] parallel_3.3.3     pbkrtest_0.4-6    
[13] grid_3.3.3         nlme_3.1-131      
[15] mgcv_1.8-17        quantreg_5.29     
[17] DBI_0.5-1          MatrixModels_0.4-1
[19] lme4_1.1-12        Matrix_1.2-8      
[21] nloptr_1.0.4       RSQLite_1.0.0     
[23] gsubfn_0.6-6       Matching_4.9-2    
[25] sp_1.2-3           SparseM_1.74      
[27] chron_2.3-47       sqldf_0.4-10      
[29] proto_1.0.0