Adding a link in Application Navigator doesn't work

I just try to add an application in Application Navigator (http://server/plugins/servlet/customize-application-navigator). On clicking Add I get "Sorry, something went wrong. Your changes are not being saved.".
In atlassian-confluence.log :

2015-04-07 20:29:15,621 ERROR [http-bio-10.20.12.102-83-exec-10] [common.error.jersey.ThrowableExceptionMapper] toR░
esponse Uncaught exception thrown by REST service: org/apache/commons/validator/routines/UrlValidator
– referer: http://server/plugins/servlet/customize-application-navigator | url: /rest/custom-apps/1.
0/customapps | userName: SDorogin ░
java.lang.NoClassDefFoundError: org/apache/commons/validator/routines/UrlValidator
at com.atlassian.plugins.custom_apps.rest.data.validation.UrlFieldValidator$Jira.isValid(UrlFieldValidator.
java:211)
at com.atlassian.plugins.custom_apps.DefaultCustomAppService.checkField(DefaultCustomAppService.java:379)
at com.atlassian.plugins.custom_apps.DefaultCustomAppService.create(DefaultCustomAppService.java:278)
at com.atlassian.plugins.custom_apps.rest.CustomAppsRestResource.create(CustomAppsRestResource.java:183)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.atlassian.plugins.rest.common.interceptor.impl.DispatchProviderHelper$ResponseOutInvoker$1.invoke(DispatchProviderHelper.java:234)

It fails on adding ANY link. Even 'google.com'.