An error along the lines of "end of file before message length reached" or "Error fetching URL" can indicate an issue with your network connection, for example a slow Internet connection. You can attempt to fix the situation by adjusting the "Maximum concurrent downloads" setting within the app.

Not all Internet issues that pop up can be fixed with this, though. In some cases, there might be a general problem with the Internet service provider, Wi-Fi router, or other extra things causing delays or interruptions between your device and the Internet. If fine tuning the maximum downloads value still fails to resolve the issue - or worse still, causes your connection to cut from time to time - then you might want to contact your ISP or router manufacturer for assistance with persistent network problems.

Using a VPN on your system may also help fix this. Sometimes, specific ISPs or countries get blocked from using certain services that the Modrinth App requires, like Microsoft's servers.