Interestingly, it was not the page cache that needed to be flushed to fix the problem.
As I suspected, the MySQL database hadn’t completely cleared the entries. Your diagnoses was correct, in that invalid web addresses likely caused the issue. My client accidentally loaded descriptions in the website link field.
Unfortunately, I did have to go directly into the database to clear all tables with “wp_connections” as the prefix. However, once I did that, the problem was resolved.
I’ll just warn him to be sure not to mismatch the fields in the future.
One last question though — what character should I use when uploading an entry that has multiple categories? Thanks!