I use Exm Subscription Form on my site. When I click "Submit" nothing happen and I see in log this exception:
11:47:00 ERROR unhandled exception thrown while processing task '0x479FA17B72734C448A640E959089B01F' from task pool 'sent_message_task_pool'. The task has not reached the maximum number of Attempts to process a failing task and is postponed. Exception: System.FormatException Message: The input is not a valid base-64 string, since they in the voids containing a non-base 64 character, more than two spaces or a character that is not valid. Source: mscorlib at System.Convert.FromBase64_Decode (char * startInputPtr, Int32 inputLength, Byte * startDestPtr, Int32 destLength) at System.Convert.FromBase64CharPtr (char * inputPtr, Int32 inputLength) at System.Convert.FromBase64String (String s) at Sitecore.Modules.EmailCampaign.Core.Crypto.AuthenticatedAesStringCipher.TryDecrypt (String encryptedmessage) at Sitecore.EmailCampaign.Cm.Tasks.MessageEventBaseTaskProcessor.OnProcess (Short Running Task task, CancellationToken CancellationToken) at Sitecore.ExM.Framework.Distributed.Tasks.Processing.ShortRunning.TaskProcessor.Process(ShortRunningTask task, CancellationToken CancellationToken) at Sitecore.ExM.Framework.Distributed.Tasks.Processing.ShortRunning.DatabaseTaskAgent.ProcessTask(DatabaseTaskDtoCollectionItem taskDto, CancellationToken CancellationToken) 11:47:01 WARN One or more tasks processed by task agent 'sent_message_task_pool' failed. The agent wants to revert to the slowest processing speed possible.
This started happening when I upgraded EXM version from 3.2 to 3.3. In 3.2 version all worked fine