Actually, Daniel, your mention of "upgrading" to LXQt has me wondering whether or not you took an LXDE version and then upgraded to an LXQt version(unsupported by Lubuntu, officially) without cleaning up all the old LXDE/GTK stuff. This could certainly lead to you using pcmanfm rather than pcmanfm-qt or otherwise using some app or apps that use GTK2, which everything seems to point to being the problem. LXQt doesn't use GTK-anything, so it should entirely be a non-issue.
Actually, Daniel, your mention of "upgrading" to LXQt has me wondering whether or not you took an LXDE version and then upgraded to an LXQt version(unsupported by Lubuntu, officially) without cleaning up all the old LXDE/GTK stuff. This could certainly lead to you using pcmanfm rather than pcmanfm-qt or otherwise using some app or apps that use GTK2, which everything seems to point to being the problem. LXQt doesn't use GTK-anything, so it should entirely be a non-issue.