Thanks Cathal, but that's not my case.
Anyway, since this error has been driving me mad, I decided to back up important information and start a new portal. Everything's fine with that fresh 4.8.2 installation.
BTW, here's what I got in Event Viewer:
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: ccae1041-34c3-402f-8f71-f13ce6ea4332
InnerException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Data.SqlClient.SqlConnection.OnError
StackTrace:
Message: DotNetNuke.Services.Exceptions.ModuleLoadException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. ---> System.Data.SqlClient.SqlException: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding. at System.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection) at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection) at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) at System.Data.SqlClient.SqlDataReader.ConsumeMetaData() at System.Data.SqlClient.SqlDataReader.get_MetaData() at System.Data.SqlClient.SqlCommand.FinishExecuteReader(SqlDataReader ds, RunBehavior runBehavior, String resetOptionsString) at System.Data.SqlClient.SqlCommand.RunExecuteReaderTds(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, Boolean async) at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method, DbAsyncResult result) at System.Data.SqlClient.SqlCommand.RunExecuteReader(CommandBehavior cmdBehavior, RunBehavior runBehavior, Boolean returnStream, String method) at System.Data.SqlClient.SqlCommand.ExecuteReader(CommandBehavior behavior, String method) at System.Data.SqlClient.SqlCommand.ExecuteReader() at System.Data.SqlClient.SqlCommand.DeriveParameters() at System.Data.SqlClient.SqlCommandBuilder.DeriveParameters(SqlCommand command) at Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.DiscoverSpParameterSet(SqlConnection connection, String spName, Boolean includeReturnValueParameter, Object[] parameterValues) at Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSetInternal(SqlConnection connection, String spName, Boolean includeReturnValueParameter) at Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSet(String connectionString, String spName, Boolean includeReturnValueParameter) at Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSet(String connectionString, String spName) at Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, String spName, Object[] parameterValues) at DotNetNuke.Data.SqlDataProvider.GetPortalsByName(String nameToMatch, Int32 pageIndex, Int32 pageSize) at DotNetNuke.Entities.Portals.PortalController.GetPortalsByName(String nameToMatch, Int32 pageIndex, Int32 pageSize, Int32& totalRecords) at DotNetNuke.Modules.Admin.Portals.Portals.BindData() at DotNetNuke.Modules.Admin.Portals.Portals.Page_Load(Object sender, EventArgs e) --- End of inner exception stack trace ---