I get the following error while I'm trying to run a Visual IVR application, with conversation:

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16:28:35:214	DEBUG	Getting conversation for : TempoAssistVoiceV2540050-34984087441

16:28:35:214	DEBUG	VPService: invoking webservice: https://10.22.50.70/axis2/services/VPAppIntfService

16:28:35:227	DEBUG	VPService: secure connection complete

16:28:35:277	DEBUG	Reading conversation variable names from file

16:28:35:281	WARN	The conversation.conversationdata file is missing, 

this may be because you have not added any varaibles to the conversation - 

java.io.IOException: Server returned HTTP response code: 503 for URL: 

http://smsassistencia.juvo.com.br:-1//TempoAssistWebV2/data/connectivity/conversation.conversationdata

16:28:35:284	ERROR	session id:10.22.50.70-2017362162833-1000 | 

EXCEPTION> 

java.io.IOException: Server returned HTTP response code: 503 for URL: 

http://smsassistencia.juvo.com.br:-1//TempoAssistWebV2/data/connectivity/conversation.conversationdata

at sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1627) 

at com.avaya.sce.pdc.appconversation.LifeCycle.loadFromUrl(LifeCycle.java:77) 

at com.avaya.sce.pdc.appconversation.LifeCycle.getConversationNames(LifeCycle.java:109) 

at com.avaya.sce.pdc.appconversation.LifeCycle.initialVariablesPosted(LifeCycle.java:201) 

at com.avaya.sce.runtime.SCEServlet.processRequest(SCEServlet.java:602) 

at com.avaya.sce.runtime.AppServlet.processRequest(AppServlet.java:76) 

at com.avaya.sce.runtime.SCEServlet.requestHandler(SCEServlet.java:243) 

at com.avaya.sce.runtime.SCEServlet.doGet(SCEServlet.java:140) 

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The file "/TempoAssistWebV2/data/connectivity/conversation.conversationdata" exists and if I remove the "-1" from the link "http://smsassistencia.juvo.com.br:-1//TempoAssistWebV2/data/connectivity/conversation.conversationdata", it works.

How can I solve it?

You shouldn't use conversion pdc for web apps. It's only meant for message apps. For saving context, you should use the context store pdc (aka Avaya Breeze/ED Integeration Workflow and Context Store pdc). There is also a sample app SaveContextToCSWeb in the install bundle you can refer to.

We don't have context store at our environment.
The context store PDC will work with the EPM conversations at the 7.1 version?

It was working... And we don't know why it stop to work, and this error is very random... Sometimes it happens, sometimes not...

And the 7.1 KT slides, it is written that it should work with HTML. From the KT slides:

Conversation data may be shared between apps of different types
Speech, HTML5, SMS, Email

EP will provide the context store of its own. Conversation relies on the context store in fact. You don't need to have an external context store. Conversation actually work pretty much the same as context store when it comes to cross channel operation (conversation across different apps on different channel). Please refer to the section Cross Channel Conversations in the DevGuide doc.
I am not sure how the HTML app is launched in your case. According to the exception, the session object is missing the request object that's supposed to be set when the session is created. Not sure why.

Sorry, I read the exception wrong. The "-1" actually comes from OD trying to resolve the port from the URL. Since there is no port specified in the incoming http request (the first request to the app), it gives your the "-1". Typically, when configuring the Start URL for the app, you would give a port also. In your case, you probably only have the host name in the URL. I am not sure how it works in other times you said. So I guess, to fix this, you would try to configure the Start URL with the port.