A technical problem hit an undetermined number of Gmail users, including paying subscribers to the Google Apps hosted software suite, locking them out of their accounts for about 15 hours on Wednesday and early Thursday.

Google first acknowledged the problem in the official discussion forums for Gmail and Google Apps shortly after 2 p.m. U.S. Eastern Time on Wednesday and declared the problem solved at almost 5 a.m. on Thursday.

A Google official posting status updates on the Apps forum wrote that the problem affected "a small subset" of Apps users, without being more specific.

More than 500,000 businesses and universities with about 10 million active users have signed up for the free and fee-based versions of Google Apps.

The problem, which also affected stand-alone users of Gmail, made it impossible for users to log in to their accounts. They got a "502 Server Error" message when they tried to log in.

In the main Google Apps Discussion Group thread devoted to this incident, administrators complained loudly about the length of the outage and the lack of status update details offered by Google officials.

A "502" error hit Gmail on July 16 as well, and also led to a long outage for affected users, according to postings in the discussion forums.

A day before this week's problem struck, Dave Girouard, president of Google's Enterprise unit, talked up Google Apps at the Pacific Crest Technology Leadership Forum.

Girouard said Google has big plans to aggressively expand the features and capabilities of the suite, while keeping the price of the Premier fee-based version at US$50 per-user annually.

Google, Salesforce.com, IBM, Amazon, Hewlett-Packard and other major vendors are big backers of cloud computing and the software-as-a-service (SaaS) model for delivering applications and computing resources via the Internet.

However, outages that leave users without access to such a basic business tool as e-mail for extended periods of time spook business and IT managers considering Web hosted software like Google Apps and cloud computing offers like Amazon Web Services.

When something goes wrong with the hardware or software in the vendors' data centers and the performance and availability of the software or computing services are affected, there is little that IT and business managers can do but wait for the problem to be solved, while their end users complain and demand information and solutions that are out of the IT department's reach.

"Seriously...It has been two hours. Can you provide us with another update? For a company with your reputation, I'm absolutely shocked at the apparent absence of customer service," wrote a Google Apps administrator on the discussion forum on Wednesday. "This amount of down time is unacceptable."

Google did not immediately reply to a request for comment.