If you are encountering this error while trying to add new entries to wallabag, you might be hitting the database_charset issue mentioned in these two github issues:
The fix of changing the charset is described in the issues, but if you set up wallabag with caprover, you just need to define a new environment variable SYMFONY__ENV__DATABASE_CHARSET and set it to utf8mb4 this will solve the issue for a caprover installation. A pull request was generated here: https://github.com/caprover/one-click-apps/pull/466