Only one user is getting the UI Container error message when performing Sage 300 tasks.
Since only one user is affected, this is likely due to a corrupt _p.ism file. Identify your shared data location, which can be found under Help / System Information:
and then browse to this location. Go into the USER folder and then go into the folder for the affected user. Locate the _p.ism file for the company where the UI Container error is happening. The prefix of the _p.ism file will be the database ID of the company. The simplest way to find out the database ID is to open the company in Sage and open any screen. The database ID will be displayed on the top bar to the left of the screen description. In the example below, the database ID is SAML14 and therefore the file we are looking for in the user's folder will be SAML14_p.ism
Rename the _p.ism file to a different filename (suggestion SAML14_p.old). The user should now logout and log back into Sage and the _p.ism file will be re-created automatically, with all the default settings. Specific settings that are contained in this file will need to be reset (such as print destination and paths to custom reports), however, the UI Container message should no longer appear.