After updating to the latest Ecwid Plugin for WordPress widget provided by Ecwid the entire backend was inaccessible. The screen was completely blank for all admin tasks - not even the wordpress top info bar was being displayed.
It may be a case of sequence, which I have assumed from the method that I have corrected it.
1) renamed the actual folders of ecwid product advisor and ecwid plugin via FTP
2) Logged in successfully as all screen data was being displayed.
3) re-renamed ecwid plugin via ftp and re-activated it in wordpress = result = all ok apart from ecwid product advisor error on chosen display page
4) re-renamed ecwid product advisor back to original name via FTP and then activated plugin - cleared cache and refreshed public page. Result = all working again now.
It may be worth having a look to see what the root cause is as other less experienced folk may have issues.