Recently I updated from php7.0 to 7.1 and I did not realise that this proces has reset the php memory limit to the default 32MB. I have now changed it to 128MB again. Do you think this caused the problem and what value do you recommend for the memory limit?
Thanks for getting back to us, I believe is because of this then caused this issue, because 32MB memory unable to handle your whole site process and the PHP image resize process.
I think 128MB is sufficient for handle your current process, because last time you switch PHP version from 7.0 to 7.1, you never hit this kind of issue before.
Perhaps you can monitor for these few days and see whether still hitting this same issue, keep us update if the issue still persists.