Thanks for your advice folks, much appreciated. I believe this has helped 
to identify the probable source of the problem. I would still need some 
further help in order to solve it though. More specifically:

@zxq9:

fstab seems to have been tampered with. It reads as follows, I can't see 
anything unusual, but maybe I am overlooking something.

#
# /etc/fstab
# Created by anaconda on Fri Sept 23 08:55:24 2011
#
# Accessible filesystems, by reference, are maintained under ‘/dev/disc’
# See man pages fstab(5), findfs(8), mount(8) and/or glkid(8) for more info
#
/dev/mapper/vg_gapqub01-lv_root / 				ext4	
defaults		1 1
UUID=56f4f74f-1327-4ef5-8888-8acf863cdfbf /boot	ext4	defaults		
1 2
/dev/mapper/vg_gapqub01-lv_home /home			ext4	defaults		
1 2
/dev/mapper/vg_gapqub01-lv_swap swap			swap	defaults		
0 0
tmpfs			/dev/shm				tmpfs	
defaults		0 0
devpts			/dev/pts				devpts	
gid=5,mode=620	0 0
sysfs			/sys					sysfs	
defaults		0 0
proc			/proc					proc	
defaults		0 0

@Steven Yellin:

You are right, this seems to be where the problem lies. When I boot 
in “single” mode and type “df”, this is what I get:

Filesystem	1k-blocks	Used		Available	Use%
		51606140	51604044		0	100%

Mounted on /dev/mapper/vg_gapqub01-lv_root

Deleting non essential data from /tmp or /var/log certainly sounds like a 
good idea. I deleted the retroclient.log from /var/log, but that didn't do 
the trick. I am unsure now what other data could be deleted without 
jeopardizing system integrity.