SDL Studio 2019 & 2021: MT Enhanced with Microsoft Azure key not working.

Dear SDL Devs and community,

we tried to install the mt enhanced plugin on a new installation of Studio 2019 and 2021. It seems that there was a chance in the Azure API regarding HTTP headers. We followed the installation instructions on:

https://community.sdl.com/product-groups/translationproductivity/w/customer-experience/1271/microsoft-translator-credentials-for-mt-enhanced-app

We are using the key in other software without problems, so the problem seems to be in the mt enhanced plugin

Studio Versions:

15.2.8.3007

16.1.3.4096

Plugin Versions:

4.3.12

5.3.12

Exception occuring in SDL logs (german system language) besides a 401 (unauthorized) warning in the Studio 2021 UI:

ERROR [2021-02-05 11:15:19,634] Sdl.TranslationStudio.Editor.Translation.TranslationMemorySegmentSearch - Das Format des Werts "<null>" ist ungültig.
System.FormatException: Das Format des Werts "<null>" ist ungültig.
bei System.Net.Http.Headers.HttpHeaderParser.ParseValue(String value, Object storeValue, Int32& index)
bei System.Net.Http.Headers.HttpHeaders.ParseAndAddValue(String name, HeaderStoreItemInfo info, String value)
bei System.Net.Http.Headers.HttpHeaders.Add(String name, String value)
bei Sdl.Community.MtEnhancedProvider.MstConnect.ApiConnecter.Translate(String sourceLang, String targetLang, String textToTranslate, String categoryId, String format)
bei Sdl.Community.MtEnhancedProvider.MtTranslationProviderLanguageDirection.LookupMst(String sourcetext, MtTranslationOptions options, String format)
bei Sdl.Community.MtEnhancedProvider.MtTranslationProviderLanguageDirection.SearchSegment(SearchSettings settings, Segment segment)
bei Sdl.Community.MtEnhancedProvider.MtTranslationProviderLanguageDirection.SearchTranslationUnit(SearchSettings settings, TranslationUnit translationUnit)
bei Sdl.Community.MtEnhancedProvider.MtTranslationProviderLanguageDirection.SearchTranslationUnitsMasked(SearchSettings settings, TranslationUnit[] translationUnits, Boolean[] mask)
bei Sdl.LanguagePlatform.TranslationMemoryApi.Cascade`1.SearchSegmentAndSubSegmentTranslationUnitsMaskedCommand.Execute(T cascadeEntry, SearchSettings searchSettings)
bei Sdl.LanguagePlatform.TranslationMemoryApi.Cascade`1.ExecuteSearchCommand(SearchCommand`1 searchCommand, SearchSettings searchSettings, Int32 searchResultsCount, IEnumerable`1& cascadeMessages)