You cannot logon to the Citrix GUI but you can logon to the CLI via SSH or Console without issues.
One possible reason is due to very limited space in the Citrix ADC disk.
Check the following directories in the Citrix ADM appliance for excessive storage size. More specifically check the /var/log occupied disk space and cross check the configuration of the Citrix ADM and Citrix ADC logging mechanism can be such that all available space is filled up by log files. In some cases the VAR/log may be huge as the ns.log files having large amount of data causing ns.log to take all the available space from var directory.
The du command recursively searches directories and returns the number of blocks or bytes each directory and all of its sub-directories are using.
- du -h
- du -ksh /var/*