Error message: Database could not be opened (Multiterm)

I am stuck with this error message (see attached file) and cannot create new entries in Multiterm 2019. I have been working the whole day without any problems and suddenly the error appeared. I have then made the update to the 2021 version of Studio and Multiterm but this didn’t change the situation. I can open Multiterm, I can search for entries an I was able to create one new entry and the next new entry had the error message again.

sdlerror-202159-8h38m28s.sdlerror.sdlerror.xml
<SDLErrorDetails time="09.05.2021 08:38:44">
  <ErrorMessage>Datenbank '' konnte nicht geöffnet werden. Entweder wird die Datenbank nicht von Ihrer Anwendung erkannt, oder die Datei ist beschädigt.</ErrorMessage>
  <Exception>
    <Type>System.Runtime.InteropServices.COMException, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</Type>
    <ErrorCode>-2147467259</ErrorCode>
    <HelpLink />
    <Source>MultiTerm160.Server.Termbase16.0</Source>
    <HResult>-2147467259</HResult>
    <StackTrace><![CDATA[   bei Sdl.MultiTerm.TMO.Interop.TermbaseSearchClass.Execute()
   bei Sdl.MultiTerm.Client.UI.Viewer.ScriptingHost.Search(String query, String index, Int32 maxHits, Boolean returnIDs, Boolean isFromDefaultLayout)]]></StackTrace>
  </Exception>
  <Environment>
    <ProductName>SDL MultiTerm</ProductName>
    <ProductVersion>MultiTerm16</ProductVersion>
    <EntryAssemblyFileVersion>16.1.1.541</EntryAssemblyFileVersion>
    <OperatingSystem>Microsoft Windows 10 Pro</OperatingSystem>
    <ServicePack>NULL</ServicePack>
    <OperatingSystemLanguage>1031</OperatingSystemLanguage>
    <CodePage>1252</CodePage>
    <LoggedOnUser>DESKTOP-SRF9G89\kzimmermann</LoggedOnUser>
    <DotNetFrameWork>4.0.30319.42000</DotNetFrameWork>
    <ComputerName>DESKTOP-SRF9G89</ComputerName>
    <ConnectedToNetwork>True</ConnectedToNetwork>
    <PhysicalMemory>11968168 MB</PhysicalMemory>
  </Environment>
</SDLErrorDetails>
PDF Could anyone please give me some advice? Thanks Katharina