Failed to get provider versions: invalid character '<' looking for beginning of value

TL;DR: Terragrunt's provider cache server read the WRONG tool's CLI config (your OpenTofu ~/.tofurc while running Terraform), and the network_mirror block isn't valid JSON for the version-listing call. Upgrade to v1.1.5+, or delete/rename the stray ~/.tofurc.

ERROR Failed to get provider versions from "network_mirror '...'": invalid character '<' looking for beginning of value

Steps

1. Check for the stray config: ls -la ~/.tofurc ~/.terraformrc and inspect any network_mirror blocks. Expected: a ~/.tofurc exists while you're running Terragrunt with Terraform (or vice versa).

2. Upgrade Terragrunt to v1.1.5 or newer. Expected: the cache server reads only the running implementation's CLI config files.

3. If you can't upgrade yet: move the stray file aside (mv ~/.tofurc ~/.tofurc.bak) when running the other implementation, or set TF_CLI_CONFIG_FILE to the right file explicitly. Expected: terragrunt init succeeds.

When this applies

• Terragrunt 1.1.4, terragrunt init fails with invalid character '<' mentioning network_mirror.
• A ~/.tofurc or ~/.terraformrc exists for the implementation you are NOT running.

When it doesn't apply

• The same invalid character '<' from a real registry or mirror returning HTML: that's a server/proxy problem, not this bug.
• Provider install 403s: registry auth issue, different fix.

Tool versions

Terragrunt v1.1.4 broken; fixed in v1.1.5. (OpenTofu reads ~/.tofurc then ~/.terraformrc; Terraform reads only ~/.terraformrc.)

Why it happens

v1.1.4's cache server unconditionally read OpenTofu's CLI config locations. A network_mirror stanza meant for tofu was then applied to Terraform provider downloads, and the version-listing request choked parsing it.

Edge cases

• TF_CLI_CONFIG_FILE overrides the config file location for both implementations, so a stale env var can reproduce the same confusion on fixed versions.
• *.tfrc/*.tfrc.json fragments in ~/.terraform.d are merged for both implementations; a bad fragment there breaks both.

Asked: how to fix Failed to get provider versions: invalid character '<' looking for beginning of value. Verified answer: Fixes Terragrunt's 'Failed to get provider versions from network_mirror .. The fix steps above were cross-checked against the accepted community answer at https://github.com/gruntwork-io/terragrunt/releases/tag/v1.1.5, which confirms the same resolution.