Update README.md

README.md

@@ -102,3 +102,36 @@ If exists: Checks if IP matches. If not, performs PUT (Update).
 
 If missing: Performs POST (Create).
 
+### ERROR
+This error (400 Invalid API key format) means that the IONOS server rejected the key because its structure is incorrect.
+The most common causes are:
+Extra spaces (accidentally copied when pasting).
+Manually adding a dot in the config file (the script adds the dot automatically).
+Using the wrong type of key (Cloud API key instead of Hosting/DNS API key).
+Here is how to fix it step by step.
+```
+sudo nano /opt/ionos-ddns/config.json
+```
+Make sure the quotation marks are placed directly against the characters, with no spaces.
+WRONG (note the spaces):
+```
+"api_prefix": " 23498723498 ",
+"api_secret": " a1b2c3d4 "
+```
+CORRECT:
+```
+"api_prefix": "23498723498",
+"api_secret": "a1b2c3d4"
+```
+STEP 2: Do not add a dot in config.json
+The Python script contains this line:
+```
+"X-API-Key": f"{prefix}.{secret}"
+```
+It automatically joins these two values with a dot.
+If you entered in config.json something like:
+prefix: "myPrefix."
+then the script sends:
+"myPrefix..secret" (two dots), which causes the error.
+api_prefix: Only the first part of the key.
+api_secret: Only the second, long part of the key.