cups doesn't properly support Epson EPL 5900L as an external program, ijs_server_epsonepl is required by the gs filter.
I got the printer working by
1) downloading http://prdownloads.sourceforge.net/epsonepl/epsoneplijs-0.4.0-1.fc2.i386.rpm?download
2) converting it to a .deb with alien
3) installing it with dpkg -i
Please consider adding epsoneplijs to Ubuntu.
Here is the CUPS log for your reference:
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D [07/Apr/2006:17:02:26 +0300] [Job 13] foomatic-gswrapper: gs '-dBATCH' '-dSAFER' '-dNOPAUSE' '-sProcessColorModel=DeviceGray' '-dBitsPerSample=1' '-sDEVICE=ijs' '-sIjsServer=ijs_server_epsonepl' '-dIjsUseOutputFD' '-sDeviceManufacturer=Epson' '-sDeviceModel=EPL5900L' '-sIjsParams=EplFlowControl=off,EplDpi=600,EplDensity=3,EplRitech=on,EplTonerSave=off,' '-sOutputFile=/dev/fd/3' '/dev/fd/0' 3>&1 1>&2
D [07/Apr/2006:17:02:26 +0300] [Job 13]
D [07/Apr/2006:17:02:26 +0300] [Job 13] Closing renderer
D [07/Apr/2006:17:02:26 +0300] [Job 13] ESP Ghostscript 815.01 (2005-09-22)
D [07/Apr/2006:17:02:26 +0300] [Job 13] Copyright (C) 2004 artofcode LLC, Benicia, CA. All rights reserved.
D [07/Apr/2006:17:02:26 +0300] [Job 13] This software comes with NO WARRANTY: see the file PUBLIC for details.
D [07/Apr/2006:17:02:26 +0300] [Job 13] sh: ijs_server_epsonepl: command not found
D [07/Apr/2006:17:02:26 +0300] [Job 13] ESP Ghostscript 815.01: Can't start ijs server "ijs_server_epsonepl"
D [07/Apr/2006:17:02:26 +0300] [Job 13] **** Unable to open the initial device, quitting.
D [07/Apr/2006:17:02:26 +0300] [Job 13] renderer return value: 1
D [07/Apr/2006:17:02:26 +0300] [Job 13] renderer received signal: 1
D [07/Apr/2006:17:02:26 +0300] [Job 13] Process dying with "Possible error on renderer command line or PostScript error. Check options.", exit stat: 3
Note: the original reporter indicated the bug was in package 'cups'; however, that package was not published in Ubuntu.
This bug is duplicate of #34647.