Caching doesn’t work – how to debug/fix?

WP 4.8.1 with WooCommerce, SuperCache 1.5.5 with default settings.
As far as I see, caching doesn’t work – there is no new files into wp-content/cache.
Into debug log I see this:

18:16:59 790 /product-category/collections/summer-products/?utm_source=facebook&utm_medium=cpc&utm_campaign=summer_labor_day_33_9-3&utm_content=make_summer_stay_9_14_ Buffer is blank. Output buffer may have been corrupted by another plugin or this is a redirected URL. Look for text ‘ob_start’ in the files of your plugins directory.
18:16:59 790 /product-category/collections/summer-products/?utm_source=facebook&utm_medium=cpc&utm_campaign=summer_labor_day_33_9-3&utm_content=make_summer_stay_9_14_ https://example.com/product-category/collections/summer-products/?utm_source=facebook&utm_medium=cpc&utm_campaign=summer_labor_day_33_9-3&utm_content=make_summer_stay_9_14_ – Page not cached by WP Super Cache. Blank Page. Check output buffer usage by plugins.
18:16:59 790 /product-category/collections/summer-products/?utm_source=facebook&utm_medium=cpc&utm_campaign=summer_labor_day_33_9-3&utm_content=make_summer_stay_9_14_ No closing html tag. Not caching.
18:16:59 790 /product-category/collections/summer-products/?utm_source=facebook&utm_medium=cpc&utm_campaign=summer_labor_day_33_9-3&utm_content=make_summer_stay_9_14_ https://example.com/product-category/collections/summer-products/?utm_source=facebook&utm_medium=cpc&utm_campaign=summer_labor_day_33_9-3&utm_content=make_summer_stay_9_14_ – Page not cached by WP Super Cache. No closing HTML tag. Check your theme.

Any ideas?
TIA,
Vitaly