The whole error text is:
<?xml version="1.0"?>

-<SDLErrorDetails time="17/03/2018 15:34:35">

<ErrorMessage>An unexpected error has occurred while loading the application plug-ins.</ErrorMessage>

-<Exception>

<Type>System.Xml.XmlException, System.Xml, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>

<LineNumber>1</LineNumber>

<LinePosition>1</LinePosition>

<SourceUri>file:///C:/Users/Pernilla/AppData/Roaming/SDL/SDL Trados Studio/9.1.0.0/plugincache.xml</SourceUri>

<HelpLink/>

<Source>System.Xml</Source>

-<StackTrace>

-<![CDATA[ in System.Xml.XmlTextReaderImpl.Throw(Exception e)
in System.Xml.XmlTextReaderImpl.Throw(String res, String[] args)
in System.Xml.XmlTextReaderImpl.Throw(Int32 pos, String res, String[] args)
in System.Xml.XmlTextReaderImpl.ThrowInvalidChar(Int32 pos, Char invChar)
in System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace()
in System.Xml.XmlTextReaderImpl.ParseDocumentContent()
in System.Xml.XmlTextReaderImpl.Read()
in Sdl.Core.PluginFramework.XmlPluginCache.Load()
in Sdl.Core.PluginFramework.PluginManager.CreateDefaultPluginCache()
in Sdl.Desktop.Platform.Implementation.Studio.LoadPlugins()]]>
</StackTrace>

</Exception>

-<Environment>

<ProductName>SDL Trados Studio</ProductName>

<ProductVersion>9.1.0.0</ProductVersion>

<EntryAssemblyFileVersion>9.1.2307.8</EntryAssemblyFileVersion>

<OperatingSystem>Microsoft Windows 10 Pro</OperatingSystem>

<ServicePack>NULL</ServicePack>

<OperatingSystemLanguage>1040</OperatingSystemLanguage>

<CodePage>1252</CodePage>

<LoggedOnUser>Pernilla-PC\Pernilla</LoggedOnUser>

<DotNetFrameWork>2.0.50727.8825</DotNetFrameWork>

<ComputerName>PERNILLA-PC</ComputerName>

<ConnectedToNetwork>True</ConnectedToNetwork>

<PhysicalMemory>8274428 MB</PhysicalMemory>

</Environment>

</SDLErrorDetails>

I have tried to restart the PC, and to repair Trados Studio 2009 via the repair button, but it still don't work.

Thanks for helping,

Pernilla