Those are called “500 errors” and are due to the server being maxed out on resources, most importantly RAM or PHP Memory. Typically your host will have a “Monitoring” panel where you can check the current % of the load on your servers CPU and RAM, you don’t really want either of them near 100% all the time, if they are you have three options in this case:
A good rule of thumb is 1GB of RAM per 3000 records on the site, more on that in CT IDX Pro > Server Health section of the documentation.
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