This error message suddenly showed up on one of my router when I tried to save its running configuration.

service compress-config allowed me to workaround this buffer problem :

router#wr mem
Building configuration...

% Warning: Saving this config to nvram may corrupt any network management or security files stored at the end of nvram.
Continue? [no]: no
% Configuration buffer full, can't add command:
************************************************************************

%Aborting Save. Compress the config.[OK]
router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
router(config)#service compress-config
router(config)#exit
router#wr mem
Building configuration...
Compressed configuration from 21787 bytes to 7991 bytes[OK]