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, provide a Debug ID for Zotero startup through reproducing the problem, using the Help → Debug Output Logging → "Restart with Logging Enabled…" option.
Upgrade Storage
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, provide a Debug ID for Zotero startup through reproducing the problem, using the Help → Debug Output Logging → "Restart with Logging Enabled…" option.