R broken after upgrade to 2.15.3 (Ubuntu 12.10, 64bit)

Hi,

I am using R on Ubuntu 12.10 (64bit). This morning, Ubuntu's software
updater automatically installed updates to R's base system (version 2.15.3;
via the CRAN PPA). Now R does not work anymore. Here is what I get when I
simply enter "R" on the shell prompt:

bodenhof FUKUOKA~>R
cannot find system Renviron
Error : .onLoad failed in loadNamespace() for 'utils', details:
call: options(op.utils[toset])
error: invalid value for 'editor'

R version 2.15.3 (2013-03-01) -- "Security Blanket"
Copyright (C) 2013 The R Foundation for Statistical Computing
ISBN 3-900051-07-0
Platform: x86_64-pc-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under certain conditions.
Type 'license()' or 'licence()' for distribution details.

Natural language support but running in an English locale

R is a collaborative project with many contributors.
Type 'contributors()' for more information and
'citation()' on how to cite R or R packages in publications.

Type 'demo()' for some demos, 'help()' for on-line help, or
'help.start()' for an HTML browser interface to help.
Type 'q()' to quit R.

Error : .onLoad failed in loadNamespace() for 'utils', details:
call: options(op.utils[toset])
error: invalid value for 'editor'
In addition: Warning message:
package "methods" in options("defaultPackages") was not found
Error : .onLoad failed in loadNamespace() for 'utils', details:
call: options(op.utils[toset])
error: invalid value for 'editor'
In addition: Warning message:
package ?utils? in options("defaultPackages") was not found
Error : .onLoad failed in loadNamespace() for 'utils', details:
call: options(op.utils[toset])
error: invalid value for 'editor'
In addition: Warning message:
package ?stats? in options("defaultPackages") was not found
During startup - Warning message:
package ?methods? in options("defaultPackages") was not found

Basic operations work, but the help system is defunct and no add-on packages
can be loaded. It seems that the utils package is missing or cannot be
loaded (for a reason that is specified, but which I do not understand).
There seems to be a fundamental problem. Any ideas what is wrong?

Thanks in advance,
Ulrich