Simply edit both the /etc/imscp/imscp.conf and the /etc/imscp/imscp.old.conf files and set the DEBUG parameter value to 1.

For versions already released:

Follow the procedure above and revert it back after.

For master branch:

Just run the script with the debug command line option. (type perl <scriptname> -?) for more information about available command line options.

The logs in both cases should be in the folder /var/log/imscp/, since there are several modules, you will have to check various files. Most of the information on failed upgrades/installs would be in those files.

/var/log/imscp/imscp-setup.log
/var/log/imscp/imscp-rqst-mngr.log