Error storing a hdf5 file with a list of strings

It's not clear (to me) why you aren't getting 2 fields in your AcqFlags dataset. I was able to get your code segment to work with a small modification. (I am using h5py 2.9.0. A new dtype was added to h5py 2.10.0 for variable length strings. That change is noted with comments in the dt= declarations. This is not an error in your code.) See below.

import h5py, numpy as np
import time

def storeFlagsFile(FLAGS_F, file_name, t0, text, ID):
    if not FLAGS_F:  # this flag doesnt work for mulitple users
        with h5py.File(file_name, "r+") as f:
            data_content = np.array([np.round(time.time() - t0, 3), text])
            asciiList = np.array([str(n).encode("utf-8", "ignore") for n in data_content]).reshape(1, 2)
            #dt = h5py.string_dtype(encoding='utf-8') # for h5py 2.10.0
            dt = h5py.special_dtype(vlen=str)   # for h5py 2.9.0
            dset = f[str(ID)].create_dataset('AcqFlags', data=asciiList, compression="gzip", chunks=True, maxshape=(None, 2), dtype=dt)
            FLAGS_F = 1
    else:
        with h5py.File(file_name, "r+") as f:      
            data_content = np.array([np.round(time.time() - t0, 3), text]) 
            asciiList = np.array([str(n).encode("utf-8", "ignore") for n in data_content]).reshape(1, 2)
            f[str(ID)+'/AcqFlags'].resize((f[str(ID)+'/AcqFlags'].shape[0] + 1), axis = 0)
            f[str(ID)+'/AcqFlags'][-1:] = asciiList

file_name = 'SO_62064344.h5'
ID = 122
with h5py.File(file_name, 'w') as f:
    f.create_group(str(ID))

storeFlagsFile(False, file_name, 4.412, 'a', ID)       
storeFlagsFile(True, file_name, 5.412, 'b', ID)       
storeFlagsFile(True, file_name, 6.412, 'c', ID)       
storeFlagsFile(True, file_name, 8.226, 'd', ID)     
storeFlagsFile(True, file_name, 9.773, 'e', ID)  

Additional thoughts/observations:

1. I noticed you are storing the time value as a string. Is that what you want? HDF5 and h5py can store different datatypes in each field/column, so you can mix floats and and strings if you want. It requires a different dtype (like a record array).
2. You use FLAGS_F as a flag to create the AcqFlags dataset. You can simplify this to test for existence, or use require_dataset.
3. You are adding 1 row at a time to a resizable dataset. This is OK for "smallish" datasets, but could be a performance problem if you create 10e6 rows 1 row at a time.

4. If you are interested, I answered other questions that shows how to do #2 and #3 above. You might find one of these answers helpful:

   • How to configure maxshape argument for H5 and append to file?
   • Resizing and storing dataset in .h5 format