There's also a stack trace that doesn't really fit in the screen shot

[FileLoadException: Could not load file or assembly 'Newtonsoft.Json' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)]

[FileLoadException: Could not load file or assembly 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)]
System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +0
System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) +232
System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean forIntrospection) +113
System.Reflection.RuntimeAssembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +23
System.Reflection.Assembly.Load(String assemblyString) +35
System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +49

[ConfigurationErrorsException: Could not load file or assembly 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)]
System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +762
System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() +259
System.Web.Configuration.CompilationSection.LoadAssembly(AssemblyInfo ai) +167
System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) +238
System.Web.Compilation.BuildManager.GetPreStartInitMethodsFromReferencedAssemblies() +78
System.Web.Compilation.BuildManager.CallPreStartInitMethods(String preStartInitListPath, Boolean& isRefAssemblyLoaded) +334
System.Web.Compilation.BuildManager.ExecutePreAppStart() +178
System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters, PolicyLevel policyLevel, Exception appDomainCreationException) +746

[HttpException (0x80004005): Could not load file or assembly 'Newtonsoft.Json, Version=13.0.0.0, Culture=neutral, PublicKeyToken=30ad4fe6b2a6aeed' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)]
System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +552
System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +122
System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +737

That looks like a different error than orignally reported. Are you using SolidCP 1.5.1 as there should be a Newtonsoft.Json.dll in C:\SolidCP\Server asp.net v4.5\bin\ which is version 13.

Well, after the update to 1.5.1, I'm now getting an error for the same component as the OP

Well, I managed to stop getting errors by replacing the following DLLs in the bin folder of the SolidCP Server (i.e. the "client" running on the managed Exchange 2019 Server.") with the exact same file versions present in the bin folder of Exchange.

System.Text.Json.dll

System.Text.Encodings.Web.dll

Microsoft.Bcl.AsyncInterfaces.dll

SolidCP Server seemed to be running versions 7.0.0, and Exchange is running 8.0.0. The errors have ceased, and I can successfully access mailbox size information, change passwords, check distribution list members, etc.

Ultimately I landed on this as the workaround for now

- Update SolidCP Server running on managed Exchange 2019 CU15 server(s) to 1.5.1 (in cases where it was not)
- Added .old extension to these three DLLs in the SolidCP Server installation's bin directory for every managed node

System.Text.Json.dll

System.Text.Encodings.Web.dll

Microsoft.Bcl.AsyncInterfaces.dll

- Restarted the SolidCP Server website in IIS