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
I found a line on which is happend this exception:
string str3 = this._cipher.TryDecrypt(eventData.ContactId);
It's look like that ContactId(equal "00000000-0000-0000-0000-000000000000") is missing
What is your Sitecore version? Is it 8.1 Update 3? if not, Did you also update your Sitecore Version? As EXM 3.3 requires Sitecore 8.1 update 3
https://dev.sitecore.net/Downloads/Email_Experience_Manager/Email_Experience_Manager_33/Email_Experience_Manager_33_Initial_Release.aspx