Thought I had found the plugin but actually the problem is back… so I am now using “Health Check”…
However, when I try to activate any plugin through Health Check with “Troubleshooting Mode” I get this message:
When enabling the plugin, “plugin name”, a site failure occurred. Because of this the change was automatically reverted…
Thank you. I solved by renaming the folded “wp-content” via sftp, renaming the “plugins” folder and seeing that the error disappeared. I then renamed the single plugins one by one till I found the one that was causing the failure.