Values of text column bigger than 255 characters are truncated to 255 characters which lead to the freeze of the sharing cart block for the user. This issue appears mainly with html text when tags are unbalanced.
example :
select text from mdl_sharing_cart;
...
<div style="text-align: justify;"> This section is for all those who want to <span style="font-weight: bold;">quickly review the bases of the English grammar.</span> <br />Be careful : if you do it all in one go, you may end up feeling sick !<br /><
...