Fix problem with Opencart website 2.1.0.1

We are an established e-commerce website and currently have a problem that we are having trouble fixing.

The Error:
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We have around 4k registered customers on our database, however for some (not all) the website crashes when they try to log in to their account. The website/server seems to "timeout".

What we have tried:
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We have tried increasing the time limit for timeout.
We have tried increasing the memory in PHP.

The only fix we have found is to delete that customer's account from the admin, and add a new customer. But for us that doesn't fix the problem really, because if other customers are having this error, they may simply not contact us about it and decide to shop elsewhere.

PHP Version: 7.2

Test Bed:
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The chosen developer would have access to the test bed, which is effectively a clone of our website on another server. They would need to then work out the solution to the error and document it to us to explain what went wrong and what the fix is.

NDA:
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The chosen developer would need to sign an NDA and data protection agreement.

Communication:
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Ideally communication can be done via Skype/telephone with us.