Out of the blue, we started getting following error on trace of one of the web service we have which consumes organisation service in turn(We were using Auto number solution available on codeplex):

The Web Service plug-in failed in OrganizationId: 07807e38-0593-e311-b686-0050568572e3; SdkMessageProcessingStepId: 14eb4ee7-c99d-e111-b463-1cc1de6e2b2d; EntityName: cal_pform; Stage: 20; MessageName: Create; AssemblyName: Xbitz.Crm.AdvancedCRMAutoNumber.GenerateAutoNumber, Xbitz.Crm.AdvancedCRMAutoNumber, Version=1.0.0.0, Culture=neutral, PublicKeyToken=b0dbcd530d1b2c81; ClassName: Xbitz.Crm.AdvancedCRMAutoNumber.GenerateAutoNumber; Exception: Unhandled Exception: System.ArgumentNullException: Value cannot be null.

Parameter name: pattern

at System.Text.RegularExpressions.Regex..ctor(String pattern, RegexOptions options, TimeSpan matchTimeout, Boolean useCache)

at System.Text.RegularExpressions.Regex.Matches(String input, String pattern, RegexOptions options, TimeSpan matchTimeout)

at ᡕ龁俄凢▥ᾑ﫡ӿ.굜ﱘ厭鴌ꌃ縛ᴉ㹃(AutoNumberSetting ᶛ핁븪쇂Ê건攇䐔, Entity ꣤᩸ၕ䧝隄旮୏, Int64 賑彏蹒準峳刉)

at Xbitz.Crm.AdvancedCRMAutoNumber.GenerateAutoNumber.Execute(IServiceProvider serviceProvider)

at Microsoft.Crm.Extensibility.V5PluginProxyStep.ExecuteInternal(PipelineExecutionContext context)

at Microsoft.Crm.Extensibility.VersionedPluginProxyStepBase.Execute(PipelineExecutionContext context)

.

On going through inner exception we found it was:

Resolution: We just downloaded the managed solution again and re-installed it(Remember to mark Overwrite customizations option):

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Hope it helps!