An internal server error on a large list upload is almost always a server resource issue with memory or a PHP process time limit (typically set at 30 seconds for most hosts) that is running out.
Your system admin needs to increase the RAM or PHP timeout for processing. On shared hosts they are likely NOT going to change this for you. In that case you will need to cut your CSV file into smaller pieces that your web host can “digest”. Store Locator Plus and WordPress have not explicit limitations. There are SLP installs that load 250,000+ locations via single upload; the servers are configured to handle the larger volume. The only issue with 100k+ locations is fetching lat/long coordinates from Google unless you already have that data in the import file.
No worries re: formatting – it works better if you copy from a web page to paste via “text mode tab” in bbPress forums. In the future you should send the info from the plugin/environment tab as it shows us what the underlying PHP environment (vs. what WordPress thinks) you have installed.