The following Link gives a review on the changes in Mapbender you can get in the WIKI

http://www.mapbender.org/index.php/Version_History

Also have a look in the trac for detailed informations:
http://trac.osgeo.org/mapbender/query?status=closed&milestone=2.5.1+release

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Changelog 2.5.1

374	WFS tables column size is too small
380	missing trsid in data SQL
28	"change personal settings" cannot be de-activated
341	add email field to forgotten password dialogue
364	Overview is not displayed correctly after Load WMC
64	Digitize Problem with 3 buttons (Stützpunkte) caused by z-index
304	Cannot load WMS with empty style name
310	Mapbender.po files contain errors
311	wrong Sonderzeichen (Encoding) in update_guis_2.4.5_to_2.5_pgsql_UTF-8.sql
312	i18n Translation does not work on all server class_locale.php
317	Layer with named styles: gui_layer_style set to null
320	treeConfGDE does not work, iframe is empty	
321	Faulty character encoding when loading WMC	
326	mapbender_setup.php - set php Version to => 5.2.0
328	Rechtswert/Hochwert mix up in print	
329	WFS spatial request: Duplicate search results
330	Spatial Request fails if layer not querylayer
331	activate element setBBOX in gui (it is needed for modul meeting point)
333	mod_exportMapImage: class_stripRequest.php => function missing
349	Add Dokumentation_mapbender25_de.pdf to folder documents
353	treeGDE2 - element_var switchwms=false shows NULL
358	treeConfGDE: mb_mapObj[ind] is not defined	
359	missing and incorrect locales in class_locale.php
361	PHP Warning: missing parameter 4 for class_bbox, class_point
362	DB connection for sld-editor in file sld_function_getusersld.php
373	featureTypeArray is not defined	
378	Missing line break in map.php causes JS bugs
295	Change personal settings	
309	After zoom to digitized geometry, highlight is not updated
325	Layer style without name: cannot display legend icon
332	element_var and line-break leads to errors
346	mod_editGuiWms.php - getCapabilities Request Link
356	"update" and "delete" button after a digitize error
313	'ZoomFull' has CloseTag img