Issue not created after clicking on blue create button of the poped-up window

[Grupo Espirita Casa do Caminho]

We had just created a new project and we were at its first screen that shows the empty form, informing that there are no issues at all in this project (it is obvious!)

Then, we tried to manually create an issue by clicking the "Create" blue button.

JIRA showed us the pop-up screen with the correct form to be filled in. In fact, we filled in all the needed fields and we clicked at the Create icon at the end of the form.

The form was closed, but JIRA did NOT create the issue after all (nor showed us the "instantaneous" small pop-up telling that the issue was created, as expected).

Then, we took a print screen (by the print screen button in the computer) and we entered in the Issue Search JIRA screen, just to confirm that the issue was NOT created (and this was true!)

Next, we tried to create a new issue again and at this time, the issue was created (and its sequential number really WAS <project prefix>-1, proofing that it was the very first issue on this project.

But this issue was born with an image attached to it, and the attached image was EXACTLY the image we had made a "print screen" some seconds before! 

Our JIRA version is 7.6.0

Can someone help with this question?

Thanks in advance.

Antonio Duarte