Start by updating to the latest version of Zotero. You're several versions behind.
Restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If that fixes it, you'll need to identify the plugin that's causing it and then report the problem to the plugin's developer. If not, we'd want to see a Debug ID in Troubleshooting Mode for reproducing it.
Upgrade Storage
Restart in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins. If that fixes it, you'll need to identify the plugin that's causing it and then report the problem to the plugin's developer. If not, we'd want to see a Debug ID in Troubleshooting Mode for reproducing it.