I could not find error code 0x80131904 on K2s website, but it looks common for SQL and Sharepoint. Check out this microsoft forum with the same error code:

Getting "Exception from HRESULT: 0x80131904" error message

Most common reason for the error is a full disk, but as you described, the hard drive is not full. But also could be allocated space for the database is full. 

One of the messages on the link refers to a missing or corrupt index.

If you need more data to locate the cause of your issue, enable page level error trace  and call stack from web config.

I hope that points you in the right direction. 

Thanks Chadg for your reply,

I've read this forum you linked, and found the post about corrupt index. After this I made a try and created a new index for a different column (single line text), and tried to search for this column in SmartObject Service Tester application in this smartobject's Get List Items method. It worked perfectly. So I tried again my original search criteria, which is a decimal column (it is interesting, that in smartobject, this column shown as decimal, not number), and it failed again. When I remove the index from this number type column, and retry the Get List Items method with this condition, also works well.

So it seems to me, that an index created on a number type column made my problem in smartobject. But how to solve it, that is the question. :)

The database allocated space is OK, autogrowth option is enabled.

Please describe how you are adding and removing the index.

I go to Site Content, choose the list. Select List Settings from List ribbon. Here I click on "Indexed columns", and on the new page I click "Create a new index", select the column from the drop-down list and click Create button.

To remove the index I go to the same page (Indexed Columns), select the index and choose Delete.

Is there anything more to do when creating?

You can try to increase the document threshold throttle From the Service Instance.

Default value is 2000.

I posted about document throttling not to long ago:

http://community.k2.com/t5/K2-blackpearl/K2-or-Sharepoint-2013-throttling-document-and-list-items-that/m-p/84743#M25721

Sharepoint does take presidence, but you can give incresing from K2 Management a shot. 

Thank you. I already tried to increase the smartobject limit, but that didn't help me.

Finally I created a table in SQL server, moved the data from SPS list to there and modified the smartobject to list  from SQL instead of SPS. I was able to create index on relevant column in SQL.

Thanks for you contributing help.

Error occurred trying to Execute (SmoSPListItem) - Microsoft.SharePoint.Client.ServerException: File Not Found
at Microsoft .Sharepoint.client.clientrequest.processResponse

What can be the reason for this issue.