As seen in the error message it is coming from Salesforce application when you try to login from Informatica (or any other application) may be due to resource unavailability at that point of time.
I could see below SFDC articles related to SERVER_UNAVAILABLE error code:
Also, there seems to be Salesforce service glitch happened on May 11th:
Please check internally with Salesforce Admin on this and if needed you can involve Salesforce Vendor Support to check more on this intermittent error.
How is the setup of your technical environment?
Are you connecting through a proxy server to the internet?
To me it looks more like a connectivity issue rather than a PowerExchange for SalesForce issue.
Maybe network administrators can do some investigation.
This looks like some intermittent Salesforce side issue.
Check if there were any intermittent issues with your Salesforce ORG during this time.
Also see if below link from Salesforce helps here: