I found the problem. When I imported the posts, the post_ID was too big to be INT in the database, so I changed it to BIGINT (as recommended elsewhere on the board). Alas, that caused a problem in two php files, b2edit.php and editactions.php. The fix was to make the following changes:

in b2edit.php:
~line 28:
param( 'post', 'integer', true );
becomes
param( 'post', 'string', true );

in editactions.php:
~line 182:
param( 'post_ID', 'integer', true );
becomes
param( 'post_ID', 'string', true );

and ~line 294:
param( 'post_ID', 'integer', true );
becomes
param( 'post_ID', 'string', true );

and ~line 385:
param( 'post_ID', 'integer');
becomes
param( 'post_ID', 'string');